You might check Tandem Cycle Works in Denver. They used to rent some tandems. I believe they are (one of) the largest tandem retailers in the US (maybe the world). They have a large selection of tandems on hand. They are located a bit south of downtown Denver - www.tandemcycleworks.com (I think)
Klein roadie, carbon proto mtn bike, burley tandem
No offense, but your's is about the 500,000th open-ended request for help buying or finding a tandem with preconceived notions about what you don't know. All you're going to get are lukewarm, highly biased, open-ended opinions from people who would love to help but struggle to understand the preconceived box you put yourself in.
Find one to try. A simple google of massachusetts bike rentals turned up this one in Bedford: http://bikewaysource.com. Don't know if you're in Massachusetts, but you get the idea..........