parents (well.. at least my parents) don't really go out much, especially not on dates. your parents would probably appreciate it if you gave them the chance to drop everything at home and go out on their own.
$100 will probably get you two tickets to something they are into.. concert/event or something. look online and see if there is anything they are into happening in your city. if they don't really do activities together, you can always just get them a gift certificate to a restaurant or something.
don't just give them cash and say "have fun.." they will appreciate it if you put the thought into something cool for them :)