I gave my parents $40 gift certificates (each) to a Harley Davidson shop (they both have Harley's). Also a few other things ... like T-shirts & such. Just find out what they like to do or where they like to go (if you don't really know already), then get them something in relation to that. I knew they liked to ride & work on their bikes, so I got them the Harley stuff.
I'm just having a time buying for my brother (23), cousin (21), and grandpa (80-something).