We're thinking of a trip to Barcelona the last week in July and will be doing alot of walking. How bad is the humidity? eg: if wet bathing suit left outside overnight - will it dry by morning? Thanks!
Barcelona is on the coast so the humidity is higher than inland. But it will vary every day during July. Yes your suit will dry if you put it in the sun.
Bathing suit probably won't dry out overnight, if you want to start out with a dry suit, then best to bring two if you're going to use it on back to back days (if you don't have access to a dryer).
Even if it doesn't dry, you can still wear it. I swim daily and often wear a damp suit. It is not unpleasant! It may even feel good to you in July.