Some[sheet]thing to keep bums dry/ warm/sheltered.

Music making equipment: machines, instruments, voices, earplugs.

Miscellanious: deckchairs, kites..whatever else tickles your fancy.

Light: If the next party will be after sunset, bring bonfires, sparklers, torches, lanterns etc to make the river glimmer.

Whetever you bring with you... take it away too.