Soft buns, lunch meats (ham, turkey, roast beef, salami), sliced cheeses, condiments (mayonnaise, mustard, butter, pickles).
And a big bowl of chips.
People can make their own small sandwich buns and eat as much or as little as they want.
If people are going to be drunk, make sure you have plenty of supplies to clean up all the vomit.
Move all breakable items out of the area.
Have the phone handy in case you need to call 911 ... parties for 18-year olds, UNLESS there are adults controllng the situation, can get violent. Especially since you WILL have uninvited guests looking for trouble who will "crash" the party.