I have taken super firm tofu and mashed it with a fork to turn it into "crumbles." Sautee it in a teeny bit of oil, diced onions, various spices, garlic, etc in a frying pan until it's browned. Then mix it into the spaghetti sauce.
Also works as scrambled "eggs," in replacement of ground beef for chili, and lots of other things.
There are tons of stuff you can do with tofu. Stir fries/curries are some of my favorites. Red curry, penang (peanut) curry, green curry, Indian style curry/korma, Chinese soy sauce curry- lots of options.