I like adpiucf's idea too. I have done gift certificates in the past to a favorite restaurant for co-workers who like to go there together for lunch periodically.
If you think they might like them, Lamy pens are also a good call. I have given those as gifts quite often. They are inexpensive, very high quality, and have a more modern and fun look to them than the more standard gift pens like Cross. Sensa pens also come in some really neat colors and are very comfortable to use.
Cake is always good too- to go with other small presents. Nothing works quite like surprising everyone with a cake and having a small impromptu celebration.
A hand written letter of heart-felt thanks would be appreciated. You could tuck a gift card inside. Or, depending on what you want to spend:
A small gift packet from Starbucks
A 2007 appointment journal
Some home-baked cookies
A picture frame
A tin of Mrs. Fields cookies or brownies
Car wash gift certificates
Movie passes for two
A CD download of songs the person likes
Hybrid Mode
