I stayed at a hostel in Germany a few years ago. Every other day, a bus stopped at the hostel. It was a company called Busabout. Apparently you pay one price and can get off or on the bus anywhere; the bus just has a schedule for stops and you need to be there to ride. Hostel stays are included. So...if you like Berlin, you might get off there and stay a week, then get back on and head to Amsterdam. Maybe you don't like Amsterdam much, so you only stay two days before getting back on the bus. The buses travel throughout Europe. You decide how long you want to stay in each place. Here's a link: http://www.busabout.com/
To answer your question, I'd love to trek the Australian outback, or the Amazon. Or South Africa. For a less wild trip, I fancy visiting Italy someday. Right now, hubby and I are considering Belize for vacation next year.