Magnetic letters! I made these because I'm starting to teach my daughter her letters.
Obviously, keep in mind that these are 3D printed, and not designed by a toy-expert, please do not give these to a kid that can't handle them, and you should always watch your kid while they are using them. Anything can be dangerous in the hands of a toddler.
These letters are designed to hold these magnets: https://amzn.to/2NkcTHk
This is the filament I recommend: https://amzn.to/2IF84Uv
You can always scale these letters to any size, but the magnet may no longer fit. I've included the original CAD files, feel free to use or modify them however you would like.