We visited Blizzard Beach in September - yes, it's lots of fun.
I personally could do the parks every day, from early morning until park closing. However, my DD's (10 & 3) would rather swim at the hotel as much as possible. On future visits, we plan to spend at least one day browsing the area and just doing whatever (putt-putt, tinkering at Marketplace at DTD, swimming, etc.). Track down some of the outlets that carry Disney or Universal merchandise and save a bundle on souvenirs. I would love to spend some time visiting some of the resorts. If you're staying on-site, find out what type of priviledges you may have at the other resorts - a day of swimming in a different pool would be fun. Just have a blast with whatever you do!