Re: Valencia
Just walking around browsing, having a coffee, people watching is nice enough. It’s very walkable and relaxed. I think the other posters have covered the top things to do, but things like the cathedral and the old river bed are a must.
Personally I would try and persuade the missus about the Mestalla. If nothing else it’s going to be a bit cooler if it’s a late kick off, often around 9.30 or 10 at this time of year, and then you can go really Spanish and have dinner (paella of course!) afterwards.
I went to a game at Levante this year but I think they are away, also walked the Grand Prix street circuit. The beach area is also worth visiting, tends to get a bit overlooked given the number of good beaches in that part of the world.
You should have a great time!