Idea:

Buy a stack of postcards, put postage on them and address them to a member of the military serving overseas. Take the stack and several pens out somewhere, get people to write supportive messages on the postcards and drop them in the mail.

I did this a couple weeks ago for a friend in Iraq, and he just received the postcards. It works especially well if you hand them out in a bar -- the messages will be much more varied and entertaining. It only costs a couple dollars, it was fun to do and it will be meaningful for anyone serving away from home.

Now go and do your good deed.