DH and I have gone on the Backstage Magic, Segway at Epcot, UnDISCOVERed Future World, Behind the Seeds tour (twice), and Backstage Safari.
1. Backstage Magic - loved it - the sheer amount of work and organization that goes into running WDW is amazing. We made stops at DHS (behind ToT)to see up close and personal where costumes are made for characters (they were making beautiful holiday gowns for the princesses to wear during the Christmas parade while we were there), the backstage area of the show at the America pavilion at Epcot (really cool) lunch at WL, the we went to the gardening area where they make the parks topiaries (awesome). Then our favorite part - the buildings behind the MK where they repair and refurbish ride vehicles AND the laundry/costume area for the CMs costumes from all over th property - fromthe country-specific costumes for each country at Epcot to costumes for resort workers, etc. I know it sounds boring but you should see the organization it takes! Then we got to go to the "first floor" of MK. Highly recommend if you love Disney and/or like to know how things work.
2. Segway at Epcot - great - we took the earliest (in the day)tour available and it was wonderful to cruise around the closed world showcase and get detailed info on the pavilions from two very knowledgable tour guides. Again, highly recommended!
3. UnDISCOVERed Future World - I know the pp didn't care for it - DH and I loved it! The guide was tremendously knowledgable about all the detail work in Future World - things I didn't notice until he pointed them out but now look at every time. We got to see the sponsor lounge at The Seas - now "owned" by Disney because the pavilion doesn't have a sponsor. The area backstage where they clean and process all the 3-D glasses for every 3-D attraction at WDW. And lots of other good information walking around onstage. Highly recommended IF you are a Future World lover and/or really into design details!
4. Behind the Seeds tour- short (one hour?) tour of the greenhouses at Living with the Land. Great if you are into gardening or have an interest in either agriculture or aquaculture. We enjoyed it both the first and second times, but I think twice is enough for the foreseeable future!
5. Backstage Safari - when DH and I went we were the only 2 people on the tour so it was awesome. We went backstage holding areas for the elephants and giraffes, then (also backstage) to see the vet center and food prep areas. On stage we got some great information about the animals on exhibition the pathways leading up to the tree of life and pagani trail. We also got to take a safari ride with our guide and she spoke about the care of the animals in the safari and how animal feeding and watering areas where built into the safari landscape. Highly recommended for animal lovers!
Hope that helps!