I love inviting friends for dinner, even if it's a last minute affair. I enjoy the company, and for the most part, your friends will be thrilled with practically anything you serve, even if it's pizza from the awesome place around the corner. That said, if you really want to wow 'em on short notice, a great summer dinner is a bruschetta bar.
This isn't really a recipe, more of a guide. Do whatever sounds good to you! Here's my method:
You're in the market. Stop at the bakery for a baguette to slice and toast. Get some dips and sauces like pesto or tapenade. Pick a cheese or two -- I usually just offer ricotta with a bit of good olive oil drizzle on top. Hit up the olive bar for (of course) olives, marinated veggies, artichoke hearts or whatever strikes your fancy. Grab a bag of arugula or something else fresh. If you really want to impress with very little effort, get some grape tomatoes or onions and roast until they're caramelized. Frozen peas are another great topper. Even get some cured meat at the deli for a more substantial meal.
Set everything out on a big cutting board or platter, using small bowls for sauces or toppings as necessary. A few plates, napkins and serving utensils, and you're set.
And most importantly, never turn down your lovely guests if they offer to bring the wine or dessert. :)