All though homemade spaghetti sauce is by far better than store bought, sometimes store bought is the only option at the time or all that is in food storage. But, don't worry. There are a few little things you can do to "spice" up a simple store bought spaghetti sauce. Here are things that I have done in the past. Sometimes it even tastes a little like homemade after adding everything.
A can of diced tomatoes.
Sprinkle in some seasonings that sound good at the time.
-Italian Seasoning, Etc.
Some freshly chopped red, yellow, or green peppers.
Thinly sliced zucchini or any other squashes.
Some freshly chopped mushrooms.
Chopped or sliced black olives.
Hopefully that gives you some ideas on things you can add in. Once you have your extras added to your sauce, be sure to heat it through at a slow and steady temperature. You don't want it to boil and you don't want it to heat so fast it pops everywhere.