OK, this is totally cheesy, but it's one of our favorite dates. After your piggy bank is full of change (what, doesn't everyone have a piggy bank for their change?), empty it out and put all the quarters and dimes in a plastic bag. Then see how far you can get by just spending that change.
(We usually end up at Chuck E. Cheese playing "Guitar Hero" and then getting fruit and cheese at the grocery store, all of which can be done with a grand total of five-ish dollars. )
You'll be surprised how much fun this can actually be. :)