Back when I was in high school, I found myself in the same situation a lot, here are some suggestions:
1) If you have a park nearby, a picnic is usually a pretty awesome idea. I never found myself lacking in sandwich material. Of course, disregard this if it is winter wherever you are.
2) Second-act a play: At intermission during any plays, a bunch of people will come out to smoke various forms of plant. Calmly intermingle yourself and your date with them, and go see the rest of the show!
3) Mooch drinks: This one only works if you and/or your date are physically attractive. Go to a very popular drinking establishment, and hang out by the bar talking about how you forgot your wallet/purse. Eventually, people will buy you drinks and start talking to you.
4) Mooch quarters: Go to your local truck stop/arcade/sports areana where there is a DDR machine. Watch people play and compliament their skills, and ask them for a quarter to play with them. My friend and I used to do this for hours at a time.
5) Backdoor a movie: This only works if you have a two-story multiplex with an exit staircase. When a movie with a large turnout gets out, walk up the stairs while everyone else is exiting the theaters. Becaue the upstairs theater exits are usually not monitored, you will be able to gain access to any movie you wish, as many times as you wish.
6) Walk around a department store, preferably one with furniture. Around Detroit, where we have many great shopping centres, we had several where you could do this. Works best with a large, but not suspiciously large, group of people.
7) Form an a cappella group. Works wonders in backyards and basements.
8) One word: Karaoke.
Hope I helped!