Our biggest fundraiser was our coin drive, which I've posted about on the forums, but you couldn't do that until school was back in session. One idea for the summer would be to get some local restaurants/bars to allow you to create a coupon for "one free drink" or "one free coffee" or whatever, and then do an "empty your pocket of change Friday" at local employers, i.e. have a team of people going around to the local office-type of workplaces, asking people to donate some change in order to receive one of those coupons...telling them about the playground and hoping that you'll receive more than just "pocket change" from them!
Also, look at hot-dog stand rentals and talk to some of the local businesses (maybe a Home Depot???) to see if they would allow you to set up a stand in their parking lot on a weekend, and sell hot dogs and soda's...
As far as a very quick high profit fundraiser, I was talking with my Aunt tonight who teaches at a Catholic School in California--they need to raise $5 mil in two years time...so I could not complain about our fundraising! Anyway, they did something called Last Man Standing, i.e. during one of the parent nights, they said something like "Everyone stand up... if you are willing to donate $10 tonight, remain standing...if you are willing to donate $20 tonight, remain standing..." and the amounts kept going up and up and up...their last man standing donated $5,000 dollars that night--wrote the check out right there and then. She said they raised about $25,000 doing this and it took about 15 minutes. Now, I know it's your summer so the issue is more of where can you get a crowd of people to be able to do this... maybe talk to the local chamber of commerce or a Kiwanis or Rotary group or a local church to see if they would allow you to come in, give the pitch for the playground, and then do the Last Man Standing idea.