I have a $30 gift certificate for the Maker Shed. What should I buy?

Link to the Maker Shed

I've had the gift certificate for a while and I just can't decide so I figured I'd see if anyone has recommendations, maybe things you've bought that you think are totally awesome. Things to know:

*I'm fine with one thing or multiple things

*I do have an Arduino so Arduino-compatible things are an option

*I think I'm more into the Make things than the Craft things, but I could go for either

Recommendations?