Circus party: i just typed loads out and lost it all when i try to link so i'll give ideas then link after.
Rather than a clown can you get a circus skills workshop to come normally they dress as clowns and demo the equipment. I went to one recently with my almost 5yr old. If you dont want to outsource it you could recreate it easily because a lot of the stuff was readily available like tin can stilts, diablo,poi, hulahoops, juggling balls or clubs, playe spinning, recreate a tightrope with a gym bench upturned if the hall has one?
For decorations, depending on how high the ceiling is you can create a big top tented effect using red/yellow or white crepe paper and a hula hoop. Balloons rather than dotted about, buy a canister if helium and balloons and fill them all and have a huuuuge mass like a balloon seller. You can recreate stations or stalls either doing circus skills games or old fashioned games like quoits,hook a duck in a paddling pool, lucky dip in sawdust, balls into a bucket, tin can alley and coconut shy.
Food, you can do this on two stalls sweet and savoury and actually could be doing yourself a big favour because instead if forcing them all to sit down you could let them come to the stall and carry on playing. (You wont lose the big cake blow moment as you can gather everyone around, maybe for a prize giving of best circus performer etc) anyway savoury food, hot dogs and burgers are ideal, if the hall has a kitchen you could possibly do oven chips and make cardboard portions of fries. Sweet, pick and mix, candy floss(you can buy it in tubs or bags cheaply) toffee apples and popcorn.
If you do the games like the tin can alley etc either do a leaderboard and a final prize giving or spot prizes like lollies etc for winners but you'll need to rope in some helpers for each "stall"