We had a fairly large gathering, 40+ adults 20+ kids and I was planning to have it at home, but we moved to the park, I know you said park isnt really suitable but..

We threw a large throw rug down on the ground and all the parents with little bubbas sat on there (it was like the size of 2 x queen bed doonas) and we had a large shaded area (any parks nearby got shade?) Its easier because you arent stressing about your house.

Then maybe you could ask everyone to bring a bottle of drink and a plate of something? Then all you have to worry about is the cake?