I absolutely love playing trail games. There are two I play very frequently.

One of them Jotto, which is a secret-word guessing game similar in many ways to Mastermind. I've written about Jotto before on my travel blog: http://withoutbaggage.com/essays/jotto

Another game that I also play frequently on the trail -- though it works best with creative people -- is a game I called Backwards, Morally-Ambiguous Fairy Tales. It's really just a variation on a common backwards story-telling game used frequently in acting classes. I've also explained the rules to Backwards, Morally-Ambiguous Fairy Tales here: http://withoutbaggage.com/essays/west-coast-trail-backwards-fairy-tales/

Finally, there is a third game I play sometimes, especially if there are children present, because it's a little easier. It's similar to the common memory game I'm Going to Grandmother's House. The way it works is as follows. One person says "I'm Hiking to the Top of the Matterhorn and I'm bringing a... guitar." Then the second person needs to add to the list, and what they add must connect to the previous word in a common way. For example, the second person might say, "...I'm bringing a guitar, pick." The the third person might say, "...I'm bring an guitar, pick, pocket." The the fourth might say, "...I'm bring a guitar, pick, pocket, calculator."

Whoever forgets something first is eliminated until only one person is left standing. For people with good memories, this can take a very long time, but the fun is really in the wacky stream-of-consciousness of it all.

Hope this helps!
-Hank
http://withoutbaggage.com