Hi,
Maybe for some decorations you could draw a couple of spongebobs, patricks, squidwards, sandys, mr. krabbs, the whole gang or print them off of a computer and hang them up, maybe a couple of those pop out pineapples, for some of the food you should have hamburgers(krabby pattys), and fries cuz thats what spongebob makes and some soda, have a trivia on spongebob and c who knows the most about him, and then get a prize for the person who gets the most. Have a drawing contest on who can draw the best spongebob. for invites you can have like a map with an X at the end which could be ur house and along the map you could write all the info on where, what time all that good stuff. Hope some of my ideas helped
Michelle