As a F/A, I'll chime in here.

We have a little joke between ourselves...under $20 is a tip (declined), over $20 is a gift!

But seriously, a letter is the best idea, but accompanied by something like chocolates is great. If I have to commute on holidays, I take Jelly Belly jellybeans for the cabin crews. One box each for the front and the back cabin, no matter where I'm sitting.