Here are a few of my favorite resources so far, I will of course be adding & removing as time goes on!
Krazy Dad A great site with thousands of free puzzles and games to print. Mazes, sudoku, kakuro, crosswords, and so on. A wide range of difficulties so I can find things for a 3 year old or for myself.
Education.com There is a limit to how many pages you can print for free each month, so I will often get ideas here & make my own pages with similar activities, varying them to target different ideas.