Stick to the nuts and maybe some high-fiber, low sugar cereal mixed together. If you are dieting you don't want the sugary dried fruits or M&Ms type trail mix. Make your own.
There is a product you can get from jayrobb.com called yammit - it is little crunchy bits of (you guessed it) dried yams, which are okay on your diet. They would be good mixed into a homemade trail mix.