The U.S. National Arboretum is an enormous, sprawling place lovely for biking or hiking. It is beautiful in every season, but fall and spring are especially stunning. You can check out the "Eagle Cam" to see the bald eagles nesting and raising eaglets there each spring, there are some fenced off old buildings from over 100 years ago when there was a bottling company and brick works on the property, and you can even head all the way to the Asian gardens to see a pagoda and a great view of the Anacostia River. BUT! What takes the cake is the columns on display that were formerly part of the Capitol Building. They're right near the visitors center and easy to get to. Do some research before you go to make the most of your trip! They even offer full moon hikes that book up FAST and for good reason-- they're amazing.
The cache is outside of the arboretum grounds. Bring your own writing utensil. ENJOY!