Well, I am a school bus driver and here's a list of things I've received in the past.
Gift cards to Dunkin Donuts (even though I don't drink coffee at all, my boys really enjoy a box of donuts on a Sunday morning )
Candles and candles and candles
Bath & Body Works lotions
Homemade treats
Money in a card that the student had made
Those gift type basket things that can be found at Walmart (whether it's a candy basket, soup basket, cocoa basket, etc)
I had this one mom make a basket of all kinds of stuff each year. It always contained a baked banana bread, a homemade jelly of some sort, homemade cookies, fudge, an ornament that the student made, a gift card to somewhere and then filled with Hershey Kisses or other holiday colored candies.
And last year, one of my favorites....I had a family of three little ones and their mom got an ornament that looks like the front of bus, in one window it says 2008 and the other window it says Ms. Pam I'm actually looking at it on my Christmas tree right now
I've also received a poinsettia one year. Even though they are pretty and it was very much appreicated, I wouldn't recommend plants. Not only do you not know if the bus driver or students are allergic, the bus driver really doesn't have much room to "store" these bigger gifts while driving. The plant they I got ended up tumbling down the stairs when I took a corner and hit a ditch.
If you get a second to ask the driver what her favorite store is, a gift card to there is also appreciated.
HTH