I think a mixture of both fun and learning is good. By mixing both, it won't be as hard getting them back into the fall routine. Kids thrive on structure, so you want to make sure that summer fun incorporates learning as well. My 7 year old daughter is attending karate camp. Sure, they go swimming and take field trips, but those things involve some sort of lesson as well.
Both. It's completely possible to play and learn at the same time. And besides just learning in general, you'll want to continue to have them work on the things they've studied through-out the school year. Otherwise they may lose some of those skills by fall and have to re-learn them.
We are doing a mix of both. My oldest just finished playschool and we had a busy year so we havent signed him up for any activities but we are doing a lot of family activities that have some learning aspects.