The Skull trap idea sounds great, but I can see a couple problems with it off the top of my head. First off, having a summoned skeleton at level 3 that can fight for you, that then explodes for 20 to 80 damage is way overpowered. Second, imagine how annoying it would be to use in a party with damage going all over the place. Third, you'd have to wait for the skeleton to be summoned. By the time he was in play, your enemies would already be in front of you.
Apparently I'm one of the few who actually likes the fireball idea. I vote yes.
I really love what Aranthys proposed for protection from missiles. I vote for his suggestion. The only thing I might change would be to remove immunity to non magic projectiles and grant an armor class bonus capped at 15 or 20. Remember, it's called protection, not immunity from magic missiles.