MAX7219 8 x 8 red LED dot matrix
These things are loads of fun and easy to use! Arduino already has a nifty library already built in. Just plug in the 5 wires and you're ready to go. I designed a simple little holder that makes mounting on the OneBot (or anything really) easy. I'll post it here for you to print on Thingiverse. If you don't have a printer....yet...I'll print ya one. Sadly it's kind'a slow to print so I'll have to charge you more for the cheap little plastic piece than the whole nicely made matrix. Here is the main code, animation module, letters module and symbols module shown in the video (click here to view on YouTube).