N.H.
I made sandwiches with regular bread used pb&j, turkey, and roast beef, then I cut with cookie cutters (borrow some if you don't have any) they looked really cute & the kids loved them. You can also cut the sandwiches into quarters/four for the adults or use croissants.
If you make your own potato & pasta salad (it's amazing how easy it is to cook) you'll also save a lot money. But I wouldn't count out Coscto either for things like potato or pasta salad. I would definitely use Costco for the water bottles, juice boxes, and plates/utencils.
I also made cupcakes (got cake decor from Michael's) so much easier than the cake thing.
The weather is also perfect for a BBQ, and there is nothing cheaper than hotdogs & frozen hamburger patties (but someone will get stuck cooking the whole time, which always sucks!).
Also, think about ditching the gift bags for the kids to take home, it's nice to give, but as an attendee I never cared to have them & my son just loaded up on cake so he never needed it. Or do a pinata instead & give out the bags for the kids to collect candy.
Good luck & if a friend offers to help, see if they can make the potato or pasta salad, that always helps!