Parc Guell in morning, Sagrada Familia in afternoon?

We paired those together on one day and it worked out really well. We took a cab to Parc Guell and then walked the 2km down to Sagrada Familia and enjoyed the walk!

I did the reverse earlier this year. I think you will be fine. It's about a 30 minute, not strenuous walk between the two.

Book an early entry time for Parc Guell - it sells out. I was their non-tourist March and it was sold out.

My take.... https://community.ricksteves.com/travel-forum/spain/infox-in-bcn-visiting-the-sagrada-familia-and-then-park-guell-or-the-other-way-around

We did Sagrada Familia at the opening time, then walked to and visited Modernista de Sant Pau, had lunch and took a bus up to Parc Guell. It was a good day in Barcelona! Assume the reverse would work out just as well.

Time of year? Some folks prefer to avoid going to Parc Guell during the hottest part of the day if they're traveling during the summer. In the winter, that would be my preferred time to be there.

we took public transpiration - subway up to Park Guell. And it was about 10 blocks of walking uphill. At one point there were outdoor escalators in the road. Walking down would make a lot more sense. It's a nice view of BCN up there, and it might be best early morning or before sunset. You can spend a lot of time at the park - there are many places to explore - it's larger than you might think, and filled with interesting corners. Both places involve a fair amount of walking and standing. I would try to schedule them on two separate days, and do other things on each day too. For example the brick "arc d'triumph" area is a fun area to walk, with coffee shops and many students. By the way, being the pickpocket capitol of EU I would have a Pacsafe or other anti-theft bag to keep your valuables.

When we went to Barcelona in April we had timed tickets for the Sagrada Familia for the opening time. I was glad we did that because
when we left at 10:30 it was getting very crowded with tour groups. We then took a taxi to Parc Guell and even though our tickets were for 11:30 they let us in at 11 am. At the ticket office at 11 there was a sign outside that said they were all sold out for the remainder of the day so I would suggest prepurchasing tickets online. We were there for approximately 2 hours and then went to the Gothic Quarter and the Picasso Museum.(we also repurchased online tickets)

I'd do it in reverse. Hit SF first thing and then walk to Parc Guell. It's a nice way to see how the locals really live.