1. He wasn't tall enough to reach the 20th button. He was able to ride down to the first floor in the morning because the first floor button is at the bottom, but when he comes home, he can't reach all the way up to the 20th button, therefore he reaches up to the 8th which I'm assuming is the highest one he can reach, and walks the stairs up the rest of the way.

2. The unopened package is a parachute I'm guessing, since falling from a plane doesn't fall under any of those categories. Possibly more than one answer to this one.

3. In the words of Oedipus, "Man." Now throw yourself in the sea, Sphinx!

4. A pothead? I dunno about this one.

5. A mushroom.