Hi Rebecca,
How cool you want to expand your Disney fun in the parks beyond the rides and attractions. Being a seasoned WDW traveler allows you to branch out without feeling that you are missing something in the parks.
Disney offers a lot of tours & special experiences - I love the tours! Click
here for a list of the ones offered at each park. I'm not sure how old the members of your family are but most of the tours have minimum age requirements as some offer a look behind the scenes & they don't want to spoil the magic for younger kids.
You can check the link listed above for minimum age requirements for all tours but here are some that are open to guests of all ages: Disney's Family Magic Tour at MK; Mickey's Magical Milestones Tour at MK is open to all ages but they recommend that it is more suited for those 10 & up; Behind the Seeds Tour at Epcot.
If the members of your family are over 16 years old, I highly recommend the Backstage Magic Tour. You get to see so many incredible things and see Imagineers busy creating magic - super cool! I also love the Keys to the Kingdom Tour at Magic Kingdom. This tour gives you insight to the history of the Magic Kingdom as well as a look behind the scenes. Both of these tours take you to the utilidors - yes - the tunnels do exist although they are not really underground - but I'll leave that to your tour guide.
The tour groups are small enough for everyone to enjoy but if you are looking for private tours for your family, contact the VIP Tour Services Department at 407-560-4033.
Wishing you many new magical experiences and pixie dust filled memories :-)