Well here in the order of priority
Wallet -so you can buy shit
house keys -so you can get back in
mobile phone -so you can ring people not at the party and annoy them
condoms-well i am sure this one is obvious but in case you get laid , also good to blow up to massive sizes then let em go in public places or play funny pranks with lol
I.D-so you can be served in bars , could be fake if you want lol
cah card/credit card -just in case the money suppl dries up
taxi card - i have one in my wallet to get home at the end of the night
overnight bag with
razer-so you dont look like a tramp
teeth brushing shizz
sleepin bag - to sleep in , may wanna get a bdouble one just in case you need to use them condoms
other washing stuff , whatever you usually use
spare cloths -just in case you throw up you are that drunk