You fellow responders are way cool. I'm liking some of the ideas for just a single summer lunch type menu for myself :-)
But my spin is on BC's contribution. You should always treat these type situations in life as a project. First of all, what is your budget (how much per head), what is the breakdown of the guests. Have you queried how many vegetarians vs. meat eaters, how many kids? I loved the nicoise salad suggestion. Just keep meats (should be two choices like beef and chicken) and veggies separate, and maybe not dressed (have a vinaigrette and a ranch type dressing available). Anyway, you probably want .5lb of meat per guest, have several salad types available, breads, possible vegetable trays, and then maybe two choices for desert. With any given budget/per head, that should give you an idea based on the number of guests what you can spend. Base your recipes and quantities on that. What about refreshments? Some kid based, some adult based. I typically would take the number 30 as a base number. Some people will eat more than a serving, some will eat less. If I'm short on budget, I would budget on 35, but if the funds were there, I'd plan for 40. Leftovers can go home with the ones that stay the longest :-)
A couple of places to give you ideas.
Party Menu Planning Made Easy
Search Results: bonappetit.com