Making sense out of timed entry in Barcelona

I went to Park Guell first. We had the first entry in the morning. I had purchased tickets for the same day at La Sagrada Familia for 3pm. We really enjoyed Park Guell, and left at about 1pm so we arrived at La Sagrada Familia around 1:30. (We took a bus from Guell to Familia) We asked if we could get in earlier and they said they are firm with times. I would imagine that most places allow 15 minutes, but I would guess nothing beyond that. We did use our wait time for some chocolate and churros and then walked around the exterior and in the park across from the entrance to La Sagrada Familia. We enjoyed the museum on the lower level of the church and could have used more time there (closing was 5pm that day) For us, I think a 2:30 time may have been perfect.

We went from Sagrada famila in the morning 9:30 to Parc Neull around 2:30. Sagrada Familia's workers were very precise and wouldn't let you in until the precise time. We stayed for about 2.5 hours including the tower which we made for 10:00am. After eating lunch around the basilica, we took the bus to the Parc. So if you schedule the morning activity earlier in the morning and the afternoon activity around 2:30-3, it should work out.

I have read nothing suggesting you have flexibility on entry times if you have a timed ticket. Those sights are packed and seem to monitor the number of people exiting and only allow folks in as space is available. If you have a 10 AM entry slot and are lined up at 10 AM, you may still have to wait at the door. Therefore, I would allow a bit of extra time on the front end of the visit.

A few of the sights (I think Casa Mila/La Pedrera and Casa Batllo, but check the websites to be sure) sell what I call "wildcard" tickets that don't lock you into a specific entry time. They cost a bit more, and I think you would still be subject to a bit of a delay at the door, depending on how quickly people are completing their visits and exiting. But you could show up at any time.

How many full days do you have in Barcelona? Which sights are you trying to visit? Unless yours is a very long stay in the city, I consider it essential to visit one of the key sights first thing every morning. That will be one place you know you can be on time without building in a lot of extra (potentially wasted) time.

Although I'm with you on the weather bit, I don't think it's safe to wait to buy your Parc Guell tickets so late that you can be confident about the weather. You risk a sell-out or having no time slots available that fit with your other plans for the day. Barcelona might still be quite warm and muggy in September, so mid-day might not be the best time for Parc Guell.

I was in Barcelona in 2015. The top sights were already crowded, but I didn't get hung up for long at the entry doors. I would allow extra time if I were revisiting the city this year. I spent about 90 minutes in each of Casa Mila and Casa Batllo. They are extremely crowded, and that affects ones ability to move rapidly through the rooms. There was an extra display about Gaudi's other buildings on an upper floor of Casa Mila, and I spent about 1 additional hour there. I don't know whether that was a temporary exhibition or permanent. Probably most people wouldn't spend as much time there as I did (a total of about 2-1/2 hours).

I also spent about 1-1/2 hours at La Sagrada Familia, but I did not ascend a tower. That would add considerably to the time you would want to spend there. The small museum is uncrowded and interesting.

There's a bus (I think #92) that runs from near the church to near Parc Guell. I do not know its frequency. The other practical option is a taxi. The nearest Metro stop to Parc Guell is quite a long walk from the park, and it is significantly uphill as you head to the park.

Time at the park probably varies more than time at the other sights. In addition to the ticketed area, there's a free-entry zone where one could spend some time. There's also a house with a separate entry line (at least 45 minutes at the time I visited, and I had an early time slot); guide books say it is skippable, and I did not wait in that line. I no longer remember how much time I spent at Parc Guell, but I would certainly figure on a rock-bottom minimum of 90 minutes, excluding transportation to and from.

The Picasso Museum is by far the most crowded art museum I've been to. (I did the Vatican Museums over 20 years ago.) Skip it if you aren't interested in early Picasso or if you cannot tolerate being shoulder-to-shoulder with far too many other visitors. If you choose to go, understand that you will need to spend a lot of time there just to fight your way into a position from which you can read the descriptive label beside each painting.

The tour of the Palau de la Musica Catalana takes a bit under an hour.

Barcelona has many wonderful sights that generally do not require advance ticket purchase. Interesting art museums include the MNAC (large) and the Miro on Montjuic. The multi-building Sant Pau Modernista Site is beautiful; two hours was insufficient there.

Personally, I enjoyed Saint Pau much more than Sagrada Familia. Our trip to Spain was in October 2017.

We were about 20 minutes early for our 10:30 admission at Sagrada Familia, and we were waved in by the gatekeepers as we loitered outside the entry chute.

The next morning at Parc Guell, we had a similar experience. Neither place was particularly crowded when we arrived, but the crowds at both increased drastically while we were there.

The magnificent stained glass windows of the Sagrada Familia are best viewed from the inside early or late in the day, because they were designed to enhance the sunrise and the sunset, with blues and greens on the east, sunrise side, and yellows and reds on the western side. Learned that during our Sept 2018 9:30 AM tour and it was magnificent, so take that into consideration. We were very moved by the cathedral (we are not really cathedral people but oh this one!) and I made my husband promise we could go back in ten years (which gives them 2 extra years beyond the anticipated 2026 completion date :) ). Yes they are strict within 15 minutes of the entry time.
https://globalgaz.com/sagrada-familia-a-must-see/img_1885/

Thank you all so much for your comments and experiences! So now, this is our current plan; does this sound doable?
Day 1: Arrive in Barcelona by plane at 4:00 pm
Jet lagged, get our bearings, check into hotel, go to TI, wander through Eixample, tapas

Day 2: Sagrada Familia as early as possible; then
Casa Mila/La Pedrera (we'll get the expensive ticket so we can go without a timed entry)
Is it possible to walk between the two? Or better to get a bus? We are 70-ish and gentle hikers.
In the afternoon, do the two city walks (Ramblas and Barric Gotic) with cathedral
Tapas

Day 3: Park Guell as early as possible; then
San Pau Hospital complex
How do we get from Park Guell to San Pau?
Rest, eat
Palau de la Musica (with tour purchased ahead)
Wander, tapas

Day 4: Train to Zaragosa and beyond

We did Park Guell in the AM with a timed entry and guided tour in English at 10:00. We decided on the morning because of the weather. it is cooler in the morning than the afternoon and everything at Parc Guell is outside. And this was in October. September should be just as hot. Then we did Sagrada Familia with a timed entry ticket of 2:30 pm and a guided tour in English at 3 pm. Also it worked better with the transportation for us to do Parc Guell first, then SGF.
Since we were staying near Placa Catalunya, we could take the #24 bus from Placa Catalunya directly to the side entrance of Parc Guell, where the tours begin. The bus stop at Parc Guell is 75 feet from the guided tour entrance. We stayed until noon, and then found a sandwich shop (Guell Sandwich park) at the top of the stairs/escalator that goes DOWN to the Vallcarca Metro station. (I would not want to walk up this, but people do it all the time.) That street is Baixada de la Gloria on the other end of the park.
We had to transfer once on the Metro at Diagonal to get the line that goes to SGF. You get out of the Sagrada Familia metro station and SGF is right there! Couldn't be easier
As for timed entry- that means timed entry. Don't be late. They won't let you in early. We got to the Picasso museum a half hour early and they would not let us in.
As you can read, we did Parc Guell in the AM and SGF in the afternoon and we were not twiddling our thumbs
We budgeted 2 hours for Parc Guell, an hour and half for transportation and lunch to SGF, which got us there with an hour to spare before our timed entry. We walked over to the park across the street to get pictures of SGF from there and popped into the souvenir store before our entry.
Don't know about swapping tickets because we were on time. I suspect it would be a hassle to do so, and since most of the time the places are SOLD out, you won't get a swapped ticket for the same part of the day. The day we were at Parc Guell, one tourist did miss the 10:00 am tour start. The next available ticket was late in the afternoon, almost 4 hours later. It would be the same at the Picasso museum, because even with timed tickets, the place is crowded.

It's a long walk between SGF and La Pedrera. 1.7 km or 1.056 miles. And you'd be doing it in the heat of September. You could take the bus, but we took the metro. Sagrada Familia Station to Diagonal Station and then a short walk down the Passeig de Gracia.

If you are splurging on expensive tickets, you might consider the night tour of La Pedrera. Timed tickets also, but we didn't buy in advance, we just walked up. The ticket takers here were more lenient. We bought our tickets before dinner and wanted the 8 pm tickets. they said we only have 10 pm tickets. We said "Look at us, we're old! We'll be asleep by 10 pm." They laughed and gave 8 pm tickets.
I like your idea of doing something a timed entry tour in the AM, and then wandering around at your leisure in the afternoon.

I think it would be important for you to take a map of Barcelona and mark the various sites you want to visit, in order to group visits that are close by, and avoid unnecessary time and effort in transit. For example, the Sant Pau hospital is a 15-minute, slightly uphill stroll from Sagrada Familia, with an excellent Italian gelato shop along the way, at least in 2016. I signed up for the English language guided tour at Sant Pau, which was worthwhile, and there were only three of us with the guide.
Rather than think of a “Ramblas Walk”, I think it would be best to plan a visit, maybe even a lunch stop, at the Boqueria market, and tolerate the Ramblas crowd as you arrive and depart the market. In the rear of the market, there are booths with prepared dishes, sort of like tapas, which I enjoyed. One of those booths, Rotisseria Ramon if I remember correctly, specializes in typical Catalan dishes.
I also took the evening tour of Casa Mila, very nice with an illuminated rooftop at the end. I got the ticket that included a very nice and relaxing sit-down dinner at the site’s restaurant next door.