I first tried this combination at a classy restaurant in Milan, Italy when I was studying abroad. It seems like such a simple salad, and it really is, but it's a fabulous side dish, or a please-make-me-one-every-day salad for people like my dad.
I like to use campari tomatoes, but these are a little expensive so use whatever smaller tomatoes they are selling at the farmers market that day or weekend (in any color--I like to use a mix of red, yellow and orange cherry tomatoes, too).
Cut bigger tomatoes in fourths, and cherry tomatoes in half. Cut the avocado in chunks as well. Toss everything with a drizzle of olive oil and balsamic vinegar. Chop up a little basil (take it from the garden if you are lucky enough to have the space for one in your backyard--I don't, but I definitely like to steal it from people that do--thanks mom). Sprinkle the basil on top, along with some sea salt.