If you've got some kind of cheese, you're good to go. Processed cheese melts easily over a plate of chips in the microwave, and a block or wedge of cheddar or something will melt well if you shred it and sprinkle it evenly. Nuke it for a minute or so to melt the cheese into one big gooey mess, then enjoy.
If you're looking to stick with the whole salsa theme though, marinara sauce can kind of substitute. If you can chop up an onion or something to give it some crunch, toss it in. A bottle of tabasco sauce or any hot pepper seasoning can give it a bit of kick. It's not the same as salsa, but in a pinch it'll do.