Camera and Journal
Documenting your trip is not only fun to do during the trip but allows you to look back on it and enjoy reflecting on your outdoor experiences. Some prefer to use only a small digital camera, but a weather-resistant journal such as those made by Write in the Rain is excellent to have in camp.
If you are on a long backpacking trip, don't begrudge the extra pound or so that a pair of sandals or light sneakers weighs. Changing into shoes that are more flexible and open than your heavy hiking boots will give your feet a chance to air out and relax.
Crazy Creek Chair
If you are on a long trip with several of your buddies, the time spent around the camp kitchen will be much more pleasurable when you have one of these chairs. They are flexible and soft and can double as a short sleeping pad if you are looking to combine comfort and necessity on a summer trip.
Bring a book to read for a few minutes in your sleeping bag each night if you are on a trip that is 2 weeks or longer. Compact paperbacks are recommended, unless you really want to trudge along with your 600-page "Collected Works of William Shakespeare."
Many explorers and trekkers have made it through expeditions without such things as an altimeter, but it can be fun to know what elevation you are at. Many altimeters have multiple functions, including detection of a high- or low-pressure air system moving through, which can give you a heads-up if there is a storm coming your way.