First time visiting Spain. We will be there in late September. I would very much appreciate recommendations for hotels in Valencia, Mallorca and San Sebastián. Staying at each for three nights. Would like a great ocean view if possible. Quiet is nice but also in a walkable part of town. Trying to stay under $450 USD per night.
Thank you!

Valencia center is not on the beach, so if you want a sea view (there is no ocean in any of those destinations) you will be far from the center. I liked the WESTIN but my last visit was about 8 years or so ago so you should check reviews online or see if others respond here with other ideas. The Westin has an indoor pool and is close to a wonderful p ublic indoor pool, which we swam in every day.

San Sebastian has many, many options. I love the MARIA CRISTINA (one of my favorites in the world, but now is part of Marriot so I'm not sure if things have changed) it has a river view but it might be above your budget.

I've also stayed at the NIZA, facing La Concha beach.

Also on that beach strip are the LONDRES Y INGLATERRA, great location and nice hotel that friends have liked a lot but I've not stayed there.

Where on Mallorca?

As far as Mallorca is concerned:

With such a short time on the island, you have a bit of a quandary. If you stay in Palma, you won't get to see much of the rest of the island without a lot of driving. We've been to Mallorca (or Majorca) many times and prefer the north of the island around Pollenca, but it's about 5k inland so no sea views, just a wonderful Spanish/Catalan feel to a great town. Again though, it's close to the north coast and Puerto Pollenca which has some good hotels.

On the west coast you have Port de Soller, a tram ride from Soller and it's old fashioned train connection with Palma. The port is a lovely place to stay. On the south west tip is Andratx, small and worth seeing, as is Sant Elm, a tiny village next to Andratx with a couple of excellent restaurants

In all, you won't see all of the island so will have to decide where sounds best to you. My own choice would be Puerto Pollenca, seeing Pollenca, Alcudia, Lluc.

Have never been to Valencia and it’s ages since I went to Palma in between terms on a charter-holiday out of Luton. Was in San Sebastián the summer before last and also loved the Maria Christina. Unfortunately the prices at the Maria Christina have more than tripled since 2021 and a few dates I checked in June had a rate close to $1000.