These crumb bars by Smitten Kitchen have become quite popular in the blogosphere, and for good reason. They're delicious, seasonal, don't require a ton of ingredients and you can make them a day ahead of time.
You can find the recipe here. I'm glad I avoided a disaster by doing research to find out that baking powder and baking soda cannot be substituted. The only change I made to the recipe was subbing raspberries for half of the blueberries. Enjoy!