Sometimes you can find bulk seeds sold for sprouting for things like radish, turnip, kale, etc. and you can go ahead and grow these out. Potatoes you just let start to sprout and you can cut them up with one or two sprouts per piece, let dry out a few days and then plant. Butternut and acorn squash, etc. are mature as purchased and, unless they are hybrids (most aren't) will come pretty true from seed. With peppers, be sure they are red...green ones likely won't have mature seed. Get some bunching onions, eat the top part, and plant about two or three inches of the base along with the
roots and they will grow right out and you will never be without green onions! A cucumber, eggplant, summer squash or zucchini will be immature as purchased and the seed won't sprout. Quite a few tomatoes are hybrids....so you won't get consistent offspring.....if you can find pear-shaped tomatoes they are probably "Roma" and
should come true. Cherry tomatoes, too. You can propagate tomato plants (as well as sweet potatoes, basil, and several other things) by cuttings from existing plants. By pruning off the suckers and rooting them, you can easily get ten or more tomato plants from a single plant.....or get suckers from someone else's plant.