We have a pool pass that sounds like your expensive one, though thankfully it's way cheaper. (Less than half the cost total, for both of us.)
The pool is definitely sozlet's default summer activity. It's not about swimming
per se, it's about the social scene. That's where most of her friends are. They actually goof around in the pool (some swimming, a lot of jumping/ diving), but then also play tetherball and basketball and hide and seek and generally goof around. The pool area is very large but fenced in and there are always adult staff at the entrance, so parents just talk to each other and let the kids go crazy, especially at this age. (Younger, we kept an eye on them more.)
If the $300 pool is where most of Mo's friends are, that sounds like it'd be worth it for social reasons alone.
I have sozlet scheduled for two all-day camps (9-4) and two half-day camps (9-noon) this summer, roughly every other week, with a trip in the middle. I might schedule her for one more. General categories of camps include nature camps (the canoe-y/ outdoorsy ones -- that's what both all-day camps will be), art camps, theater camps, zoo camps, science camps, and dance/ gymnastics camps. (Not every one of those each summer, I mix 'em up.)
A mixture of structure and free time (pool on nice days, friends over or at friends' houses on not-as-nice days) seems to work best, for both of us. I really like having intervals where she's busy and I can do my thing.