The cork catapult can be made in minutes, and be fun for hours. Use it in mini battles or make a game and see who can get the coin (or whatever else you want to launch) into a cup more times. I like this one because everyone has the stuff right now to make it. No corks; go ask a neighbor.
Rubber band or hair tie
Careful, this will launch much farther than you might suspect so maybe stay away from the windows. You might just as well launch small rubber animals or erasers onto concentric circles for different scores or though hoops!