For folks in the Tri-Corner MA/NY CT area (Say Sheffield to Sharon, and Goshen to Amenia), the Falls Village Pool may be a great place for a cool destination visit for you or others you'd like to plan to enjoy it with. Check the town recreation site for rules and fees.
They are fairly reasonable for individuals from other towns (5 dollars per person per day). For a three hour party, there's a fee of $125 so see how many folks you could invite to make that worth while.