How many people are we talking about? My answer is based on the assumption that it is a relatively low number, and thus economically viable for you.
You have a hiking website, why not send them something hiking related? And if it has your logo or URL on it even better. The first thing that came to my mind was a backpack, but I'm not into hiking, so you can probably come up with something better than me. I'm sure there are plenty of relatively inexpensive niche hiking products.
Another idea is to donate a certain dollar amount to the charity of their choice.
Here are a few other questions that may spark some ideas:
What's a great gift for awesome customers?
What is the best material to hand out at a conference/networking event besides your business card?
What is the most memorable swag you've received?
Reward customers for bugs reports?
Swag & Giveaway Advice!!