Go up to a random person and:
1) Ask what the meaning of life is. Before they have a chance to answer, say, "You're wrong! It's 42!
2) Ask for directions to Magrathea. See how many people you have to ask before someone catches on.
3) Sing a completely random song that's completely out of season, such as "Frosty the Snowman" in August..
3a) ...in a different language
3b) ...in a made-up language
4) Find someone with large/long hair and compliment them on their hair, then ask them where they bought it at, because you want one too.
5) Ask if someone has seen your friend. When they ask what your friend looks like, say, "He/she is invisible, so I'm not really too sure."
6) Find someone who has a gold ring, with no details or gemstones in it. Grab the ring, pretend to be pulling it from them, and say, "MY precioussssssss." Glare at them. "You hassss my precioussss. We wantsss it, yes we doessss very much."
7) Ask if they believe in staccato rests. If they say yes, ask them to start a campaign with you. If they say no, scream and yell at them and run off angrily.
8) When it's the middle of summer, come out in winter clothing with a scarf and gloves, etc. Comment that it's awfully chilly that day.
9) Ask, "Are you one of THEM?"
10) Scream "YOU'RE ONE OF THEM!!!!"
11) Rant about things that have nothing to do with anything, such as why the sailor senshi get their nails painted during their transformations, only for them to remain hidden under their gloves.
12) Say, "Yo! Wazzup, my homie?!" Works especially well if you're dressed as a prep or goth.
13) Find someone a lot shorter than you. Complain that they're too tall.
14) Go up to someone, act completely serious, and ask, "Can I eat you?" (thanks to constance =P)
15) Find someone who has a bottle of Evian water. State that they must be naive. If they ask why, point out that Evian spelled backwards is naive, so they must be naive for drinking it.
16) Wear a sign that says "Preps (or any other highschool label) scare me." Works especially well in areas crowded with teenagers, such as malls and dances.
17) Find someone and tell them their bra strap is showing. Works especially well if they're a man or if it's really not showing.
18) Find someone who looks worried, and say: "Don't let worry kill you, let the church help!" (thanks to olly =P)
19) Scream "Don't panic!!!" at the top of your lungs.
20) Ask how many roads a person must walk across. After they answer, say, "No, it's 42. The little white mice told me."