Nothing wrong with BBQ chicken, corn on the cob, pasta or potato salad or a green salad, and some watermelon. Buy 2 or 3 6 packs and a bakery cake and you're all set. Doesn't need to be fancy to be fun.
If you are going to BBQ several different things make smaller portions (mini burgers, sausage cut up, chicken thighs) so that people can try more than one without feeling like a pig

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