For my boyfriend's birthday I got a bunch of balloons, stuffed notes inside them(good memories, random thoughts, parts of instant messager conversations we've had) then blew them up, drove to his house at midnight and filled his jeep with 'em. Needless to say, he was quite surprised the next morning!

If you try this idea, I definitely reccomend tying the balloons together in strings of 10 or so with some gift ribbon..makes them much easier for you to put in the car and for him to take out! Plus, the idea only worked for me because i knew he doesn't lock his car.  The balloons take a while to blow up, but it's fun.