Does this sound like a good idea for a party? what could i do if it rains?

im turning 14 in a month and i was thinking of asking about 20 people( guys + girls ) to my house for a bbq/ garden kinda party for a few hours. pretty much everyone knows each other and im gonna borrow speakers to get loud music, and a chocolate fountain, and have my closet friends to stay afterwards. but im thinking what will i do if it rains, and my gardens kinda small and hilly, and my house isnt that great so any back up plan ideas? thank youu (: