These are harder than they look, because making something is the fun part and selling it is not. A teen who makes forty items before selling one has built inventory, not a business.
Baked goods. Check your state's cottage food rules first, since they govern what can be sold from a home kitchen and they vary a lot.
Candles, soap, bath products. Low cost per unit, high effort per sale. Works best with a repeat local audience rather than a marketplace.
Jewelry, keychains, stickers, prints. Very low startup cost. Very crowded online, so the winning version is local and specific, like designs for one school or one team.
3D printed items. Needs a printer, so this is a real investment. The businesses that work sell a specific useful thing to a specific group rather than novelties.
Dog treats. Cheap to make, and the customer is standing on your street already.
The rule for this whole group. Make three, sell three, then make more. Not the other way around. A teen who follows that rule learns pricing and demand in a week. A teen who makes fifty first learns neither and has spent the money.
Who this suits. A teen who genuinely enjoys making things and can be held to selling before producing more.