My first reaction is that lunch must be awfully expensive in Houston :)
Spirit notoriously figures out a way to charge for everything. Do you already have a physical ticket? If not, you might be charged $20 to print it out at the airport. You also might be charged for a seat assignment. Needless to say, if you're carrying (or God forbid, checking) a bag, that'll be extra as well.
Be sure to have your credit card handy at all times during your flight. With any sudden loss of cabin pressure, oxygen masks will drop from the ceiling, and you'll only have about thirty seconds to swipe your credit card and activate your mask before you pass out.
Also, make sure that they actually LAND in Vegas, rather than just shoving you out the door with a parachute. If you can't put up a $500 deposit for the chute, they'll eject you without it. Be sure to pee at the airport before you leave, because there might be a charge to do that on board the plane.
If it's possible to dodge ALL the traps, you'll have an only moderately unenjoyable flight, and you'll arrive with more slot machine food in your pockets than you otherwise might have. Let us know how it goes!