First time in Spain: Barcelona vs. San Sebastian?

It is cooler weather that time of year in San Sebastian. And there are less people in July compared to Barcelona. It is better as it is not really easy to get to,

Get you a hotel on top of the Mountain there http://www.monteigueldo.com/en/
That seems more honeymoon romantic than Barcelona.

But since you say you are no partier, you might want not to go there when this fabulous jazz festival takes place usually the 3rd week of July:
https://heinekenjazzaldia.eus/en/

Both cities are likely to be full of other tourists--a lot more of them in Barcelona, but they will be more spread out in that much larger city. It's true that the weather in San Sebastian is less likely to be unpleasantly warm and muggy. I think it's also less likely to be sunny, if that matters.

I find Barcelona many times more interesting than San Sebastian. However, lots of people love San Sebastian, and they cannot all be wrong. And:

• I do not drink and don't enjoy bellying up to a bar to try to buy tapas, which seems to be a very big part of the scene in San Sebastian. You say you like hanging out in bars.

• I do like art museums (thin on the ground in San Sebastian--but that won't bother you).

• I adore Barcelona's modernista architecture; perhaps you don't care about that. San Sebastian is pretty enough.

• San Sebastian's beach is absolutely stunning. Not necessarily warm enough for swimming every day (you may see people in wetsuits), but lovely to look at.

• If by "4 days" you really mean 4 nights, that's sort of short for Barcelona and could lead to a hectic visit, not necessarily what you want for a honeymoon. Four days (nights?) would be great for San Sebastian. You'd even have time for a day-trip.

One caution: Check flights in and out of San Sebastian and Bilbao to be sure they will be workable before falling in love with the idea. You won't want to spend an entire day getting to your third destination, I don't think. You may have better luck with Bilbao, which is a busier airport. Some quick Googling indicates there are hourly buses from the Bilbao airport to San Sebastian with the trip taking between 1 and 2 hours.

Thank you @acraven for always replying to questions in a polite, kind, and helpful manner- along with giving great tips and advice- it is much appreciated!!!

Happy Travels💫✨

San Sebastian (really Donostia before being given a Spanish name) is a nice town, but needs to be visited in the high summer months (July/August) because it's rainy and chilly the rest of the year. The down side, as acraven points out, it may be harder to get to and burn too much of your valuable time. There are some very good seafood places in Donostia and bars that serve great "Gintonics" (one word, the other popular drinks are beer, wine and mojitos). Potential daytrips - with a car - are Pamplona, Saint Jean Pied de Port, Bilbao, Olite Castle or a wine museum in Vitoria Gasteiz. Wineries need advanced reservations one or two days ahead, they aren't open like American wineries (maybe some are open in July?).

Barcelona is a good city to visit. I think you can cover quite a bit (not everything) in three days/four nights. Even more in four full days. I'd spend one day exploring the Barri Gothic, another exploring Modernista architechture (including Parc Guell and Sagrada Familia), a third day in town (maybe Mt. Juic?) is not too much but can also be used for a day trip to either Montserrat or some combination of Costa Brava/Girona/Figueres. For Montserrat without a car, stop by any bus station and book a seat on one of their day trips. For Costa Brava a train is easy. There may be daytrips on buses to Girona/Figueres but I haven't done it. My favorite tapas bar is El Xampanyet, most of the others are chains (IMO over-rated).