I guess I'm weird or something - but giving money always makes me squeamish - especially to adults.
Sometimes teachers get useless gifts, but appreciated, We usually try and find out what types of resturants they like and give gift certificates.
Attaching a nice card( expressing thanks and have lunch/ dinner on me seems to work.) & creative display of money has always been appreciated and they can enjoy as they please I used to think giving money was a thoughtless but...Kids like to give teachers unsual gifts. Just did a picket-fence box with fake plants and money tree with godiva chocolate coins as flower bed.