I have a friend who is looking for tours she can join without paying a high premium for single passage. She wants adventure travel hopefully hiking, kayaking and open to locations.
Check out Overseas Adventure travel, otherwise known as OAT Travel. No single up charge.
Also see about Grand Circle Travel. I don't think they have a single supplement.
mary,
She could have a look at G Adventures as they have quite a wide range of tours, including (I believe) "adventure travel". It might be difficult to find a company that doesn't charge a single supplement, but it's easy to avoid that in many cases by agreeing to room with another person on the tour. That's also true of Rick Steves tours.