Veal (or chicken) piccata/scallopini is a last minute dish but it is really fast if you have the meat already pounded thin and the lemon slices cut up, etc. If you have a rolling cart (or kitchen island open to the dining area) and a plug in griddle you could even make it a la minute at the table for a bit of a show - and, I think it is also light enough for a summer indoor dinner. Likewise a shrimp scampi meal can be thrown together a la minute. Pre-boil (slightly under) some egg noodles and dip into a pot of boiling water for a few seconds (use a slotted scoop so you are not fishing for the strays). Add a salad for garnish/the vegetable component.
If you are looking for something a bit more ethnic and oven-centric but a bit different than just your basic stew, perhaps a tagine of some kind might fit the bill.
ETA: Fondue, hotpot, hot stone cooking is a great way to prep ahead and enjoy all your time with your guests over a good long dinner. Everyone just cooks their own dinner - and it can be done winter or summer.
