A couple of these strategies are summarized below.
PlaceIt helps you create mockups of your designs to put on products. This is perfect to start your on print on demand store. They also have a new logo generator that is amazing. You can use this for an existing client or pick up new clients.
To make money: start a print on demand business using your old designs on the products
BookBolt is an online designer that will help you get started on Amazon KDP. The BookBolt Studio can create line graphs, journal pages and more. They also have a puzzle generator that creates puzzles in a few clicks. Their interior pages are perfectly sized for KDP and you can generate new books in minutes.
To make money: start a KDP business using your old designs