I went on a beautiful hike yesterday on the coastal trail in San Francisco. While I was walking I noticed a woman who was getting a little bit more benefit from her walk than me. Rather than just carrying her water bottle with her as she walked, she was doing bicep curls.
Of course! It was so obvious. If you are going to carry the additional weight of a water anyway you might as well do some arm toning in the process. So leave the backpack at home and lift water bottles as make shift weights.
The other benefit? By continually lifting the water, you'll be reminded to drink it. This way you'll finish your walk not only with some good cardio and some toning but you'll be hydrated as well!