I'd second the suggestions of shrimp. Marinated and on skewers, they take only minutes on the grill or under the broiler. You could accompany them with a rice or pasta based salad and good bread.
Another thing we like in the summer is a marinated chicken and roast pepper dish that is served at room temperature. The peppers are roasted and the chicken (breasts) poached and sliced in advance, everything is arranged on a platter (I put the chicken on top of the peppers) , add some olives and a good vinaigrette. You can do a similar thing with steak. Everything can be done in advance - then refrigerate it. It is best served close to room temperature but should not sit out long for food safety - I give it about half an hour out of the fridge.
