If you can afford to do steaks, I would say go with an assortment of steak, chicken breast, and salmon. I would find out ahead of time which of these meats each guest prefers so that you can better gauge how many pieces of each type of meat to have available. You might also want to see if anyone is a vegetarian or has any type of specific allergy or health concern so that you can make certain there is something on the menu that s/he can safely and healthfully eat.
There is so much you can do with steak or chicken. You could cook them in whole pieces or cut them into chunks to make kebabs with thick slices of onion, bell peppers, zucchini, and yellow squash (or even other vegetables, as desired). If you cook your pieces whole instead of doing kebabs, you can slice the vegetables and cook them in a grilling basket or in an aluminum foil packet. Whatever you do, make sure you brush them with olive oil first before grilling. The type of seasoning would be up to you. We keep ours simple: freshly ground black pepper, Kosher salt, and a sprinkling of garlic powder. A crowning touch of Parmigiano-Reggiano cheese (shaved or grated) is always nice.
Also, grilled corn on the cob is excellent. You can actually cook them in their husks if you soak them in ice water all day before grilling, or you can shuck and wrap them in aluminum foil to grill them. My husband and I have found a mouth-wateringly delicious way to season corn on the cob: butter (we use spray butter for the lower fat content and calories; plus, it's easier to apply), shredded Parmigiano-Reggiano cheese (you must roll your butter-coated corn in a plate of it to coat it well), and a Cajun seasoning (we use Tony Chachere's).
Another vegetable that is fantastic on the grill is asparagus. Lightly coat it in olive oil and sprinkle it with Kosher salt and freshly ground black pepper and grill to the desired tenderness. It is very easy, though you must watch it closely and turn it frequently.
And may I highly, highly recommend purchasing a fresh (never frozen, if at all possible) fillet of salmon? It tastes absolutely divine
when cooked on the grill. Leave the skin on and lightly coat the flesh with olive oil, sprinkle with salt and pepper, and toss it skin side down on the grill. (You can place it on aluminum foil if you would like, just to make it easier to transfer.) Look up cooking times online, based upon the thickness of it. We cook ours anywhere between ten to twenty minutes on the grill, taking it off when the center becomes hot. It's important to leave the skin on when cooking because if you strip it off beforehand, then you are removing the heart-and-brain-health y Omega fatty acids that are so good for you (and that also just happen to make the meat taste wonderful!). Plus, the skin ensures that the salmon stays moist and that it retains its rich flavor throughout the cooking process. No matter how big of a piece we buy in hopes of having enough for leftovers the next day (it tastes great placed cold on salads), we always end up eating almost all of it in one sitting--it's just that
good and, even better, is incredibly simple to make.
Definitely have some spring salad on hand with sliced fruits. You could do an assortment of any of the following: pear, honeydew, seedless watermelon, cantaloupe, strawberries, blueberries, raspberries, blackberries, dried raisins or cherries or cranberries, pineapple, kiwi, and/or grapes (though you do not have to be limited only to these as I know I am leaving plenty of choices out). If you slice your assortment ahead of time and place them all together in a bowl, they will produce a delicious marinade of fruit juices that your guests can spoon onto their salads when they are getting pieces of fruit. What could be better on a hot summer evening than a refreshing salad with fruit? I love salad with fruit and grilled chicken or salmon. The luscious fruits make all the dressing that you need. Of course, I would still suggest having a homemade vinaigrette on hand. Vinaigrette would complement all these meat choices when spooned over either a main course or a side salad. I also like to have crumbled blue cheese (there are many excellent varieties, such as Gorgonzola) and praline pecans (especially amaretto-flavored pecans) or walnuts to add to the salad, along with the fruit and vinaigrette.
My favorite vinaigrette is quite simple: olive oil, balsamic vinegar (raspberry-infused is my very favorite), stone-ground or Dijon mustard, minced shallots or garlic, and salt and pepper. I do everything to taste. (Different people prefer different ratios of these ingredients. Just experiment to see what you like.) You can also toss in any herbs that you like. Fresh herbs make anything special.
All the above (with the exception of slicing the fruit), require very little effort as far as prepping goes. If you are short on time, just pick up some potato salad, chilled pasta salad, or antipasto at a local deli. On a hot summer evening, a chilled side dish is more welcoming than a hot baked potato. I also echo the suggestion of serving good quality tortilla chips, guacamole (which is easy to make), and salsa. Your guests could mingle and snack on the chips and dip and the antipasto (which I mentioned earlier) while the meats are grilling.
Good luck with your planning, and try to keep it as simple as possible. You don't want to be so stressed that you cannot enjoy the company of your dear friends.