Paying the bill or leaving a tip with cash, I generally try to fold something out of the bills. Here are some ‘doodles’ I did yesterday; the tilted dollar I believe has been done (I’ll can’t find the reference now), but the rest I haven’t seen. Using US currency as an origami medium is actually quite useful; the US dollar is made out of some of the best paper available, it is dimensionally very consistent, it doesn’t rip, and it is widely available.
The $2 bill is yes, still in circulation. You can get them at any bank, you just have to ask. They are great as tips, and folded, they are even more unique.