What can i do for my 14th Birthday Party!!!???? HELP ME?!

Ok my birthday is in a few weeks and i don't know what to do for my party. I think it will be in my backyard, where we have grass, swings, a bbq area and some paving. but im open to ideas.
i can have up to 30 guests and a mixed guest list.
it will probably have to be outside though.
all i can think of is water bombs etc. or silly string but my birthday is in autumn so it might be a bit chilly for water fights. and i have no idea where to buy cheap silly string because it $5 a can at the supermarket. I have a disco ball and coloured lights too.
i have to keep the party clean, so NO kissing etc. i want to keep truth or dare away!!
any other ideas?
thanks :)
oh and where can i get cheap silly string??
thanks again =)