Have a potluck. Make it clear from the start that it's a birthday potluck and don't expect presents or cards if guests agree to bring food, paper plates, cups and napkins. How about music. Somebody must have something that can play music. Do you have a yard or a nearby public park that doesn't charge for picnics (they usually only charge for the covered area with grills if they charge at all). There are lots of old-fashioned party games that don't cost a penny like musical chairs, egg and spoon races, three legged races, pin the tale on the donkey and the like. People just need to be in a happy frame of mind. 30 might be a bit too many though, and if you can't stretch to a sheet cake you may want to postpone until you can, but like I said, you don't need much to make a party.