Tres leches is a traditional Mexican cake, so I would think of spicy food to go with it. Now, most people think of tacos whenever they hear "Mexican food" but there a loads of more fancy stuff in the mexican repertoire, that is delicious!, like chicken in green and spicy sauce, or mole. Chiles rellenos (stuffed peppers), tortilla soup, or simply guacamole...
I have a lot recipes in Spanish, but if you think of anything you would like to make, let me know and I'll see if I can get it, translate it and post for you, OK?