hi we did something similar for dd ist birthday in the garden, was thinking bbq but didnt want to be stood cooking!
we did ask everyone (every family group) that was coming to bring something, this meant a lot less prep for us, less money spent and less plates needed for serving up, plus everyone felt they were helping, but this isnt for everyone,
we did carrot sticks, cucumber sticks and celery sticks with humous and dips, nachos, salad, couscous, cheese and crackers, olives, beetroot, garlic bread (only thing that needed cooking on the day) i made up a tomato and onion pasta in sauce and let it cool as a pasta salad, i bought a load of fresh bread and made open sandwiches with nice meat from deli counter, then people could add salad and sauces as they wish as we have picky families!
feta cheese cubed with sundried tomatoes on cocktail sticks are yummy too! maybe goats cheese tart with olives on?
MIL did home made salad, nanna did quiche, friends did sausage rolls, another friend did cheese/pineapple, cheese/onion, cheese/cherry tomatoes, cheese/mini sausages,all on cocktail sticks, someone else bought muffins, i made a fruit terrine the night before, i bought some cones and ice cream and served them for dessert x
what about making wraps and slicing them up instead of sandwiches?