Take a grandparent, or two! This is my personal recommendation...being a grandmother.
We have actually used all of the above tips and had wildly susseccful, wonderful trips with varying ages of children and adults. As for the grandparent thing ( or any doting adult will do) i really think you can get the best of all worlds. I travelled with my DD, DSIL, and 8 mo DGS and another young couple a number of years ago. My DD and her husband had to take a trip in this time frame, but were reluctant to be seperated from their son for the week. I piped up, offered to pay my own way and to provide all the TLC my DGS required, while keeping him in the same zip code.
I accompanied them to the parks everyday and happily scooped up my handsome little prince whenever his parents and their friends wanted to enjoy an attraction or meal that wasn't age appropriate for him. I enjoyed the few evenings that he and I got to retire early, while the others had a late night. Back in the day, we communicated through walkie-talkies, and DD could always check in to arrange our rendezvous points. I saw the World from a totally different POV on that trip and it was one of my best trips ever. DD was thrilled to have her boy close by, and still be able to totally enjoy a couples' trip with friends.
We think we hit a winning formula, because we're are still travelling together, and have made numerous trips back to WDW and even a cruise. In fact, one week from today, we are surprising that same boy and his brother with another trip. The boys are now the high energy travellers, but the theme remains. DH and I are delighted to have any time alone with them we can whenever their parents would like to explore hand-in-hand with just one another. And we'll do lots of things all together.
Oh...we tried MemoryMaker last Sept and absolutely loved the photos and videos.