Nuts can defintely be a good thing to carry-higher the fat, so much the better. Since going on my low sodium diet I eat a 1/3 cup of no salt cashews with 1/8 cup of Craisins every day at work to keep going (there isn't a lot I can eat) I plan on eating this while backpacking this coming spring (it's a bit white out here right now!)
With you doing 20 days, weight will be a big issue, but for calories=weight, nuts are an excellent choice (especially with how dang heart healthy they are!)
I am a big propent of freezer bag style (but that is just me!) My website is www.freezerbagcooking.com - it is a free website with a very large database of recipes and concepts :-)It could give you some ideas at least!