Well, I'm an APA and IPA lover, so I would always suggest one of those! If you use high AAU hops for bittering, you can get away with less hops than you'd think.
If you've already got a blonde and a hefeweizen, the only other beer that screams "summer" at me is another wheat, if not an APA.
I hate to admit it, but I'm drinking a watermelon wheat right at this moment. It's over 90 degrees, though, and it's both my first wheat beer and my first fruit beer, all in one. The recipe is under Dude's pulldown. It was pretty cheap, because I used hops I had in the freezer (they aren't really tasted) and it used 2# of honey. You could sub corn sugar for the honey, if you wanted real cheap.
Edit- here's Dude's recipe: http://www.homebrewtalk.com/f62/bs-honey-wheat-21227/
Here are my changes:
4 lbs Wheat Malt, Bel (2.0 SRM) Grain 43.24 %
3 lbs Pale Malt (2 Row) US (2.0 SRM) Grain 32.43 %
4.0 oz Honey Malt (25.0 SRM) Grain 2.70 %
1.25 oz Crystal [3.00 %] (60 min) Hops 12.4 IBU
1.00 oz Crystal [3.00 %] (20 min) Hops 6.0 IBU
1.00 oz Crystal [3.00 %] (0 min) (Aroma Hop-Steep) Hops -
2.50 cup Watermelon Juice (Secondary 10.0 days) Misc
2 lbs Honey (1.0 SRM) Sugar 21.62 %
1 Pkgs Nottingham (Danstar #-) Yeast-Ale
I just happened to have tons of clover honey, and plenty of crystal hops, and all of the other ingredients in my basement.