Hit the trail. Going for a walk in the woods can be a totally relaxing activity. It's just the two of you and Mother Nature, so it's the perfect opportunity to be candid with each other. Since you probably won't come across many other people, you can open up and be yourself and not have to worry about impressing anyone. She'll find it romantic and will appreciate the real you.
Hiking also doesn't have to be that involved. Take a couple of water bottles and go to your local forest preserve. Find a state park that has an extensive number of trails. Be sure to pick a trail that's at a level of difficultly you both can manage, and follow the map. While being in the woods is fun and romantic, being lost in the woods is not.