Well sometimes we do money, usually the change at the bottem of my purse, though dollars are easier to slide under the pillow. We also do books or a $ store type fun thing, neck lace, (haven't gotten to a boy type gift yet for a tooth he's only 2yo).
Also an idea that would be nice now, cus my kids wake at the drop of the hat at night (though I can't get them up in the morning for school, go firgure) Is to put the tooth in a glass of water in the bathroom and in the morning the tooth fairy takes the tooth and leaves change in the glass of water. (my nieghbor does that trick)
I recived books as a child under my pillow sometimes, and to this day I still remember one of those books, (It was a picture book dictionary)I think that was for 2 teeth I had lost :)
I have also done this, we had a mix of money from around the world and we gave our 4yo at the time, a stack of coins from different countries, she thought it was pretty cool, though she couldn't spend it on anything, more of a token.
Though I have to say w/ 4 children I have forgetten to put something under the pillow. And I used the excuse "Oh my that is so not fair I can't believe she forgot!, She (tooth fairy) must have been so busy last night , or well maybe the tooth came out too close to bed time and she didn't get it on her list in time./
Anyways have fun with it!!!
P.s. Just remember if you give alot of money $5 then they will expect that or more next time too. We made that mistake w/ our 1st daughter. But she learned there was no rhym or reason to the tooth fairy's giving/payment factor.
Also My dad saved all our teeth and when we grew up (college) he gave us allll our teeth back LOL.