There are several brands of lifeboat style rations, like the Mainstay bars mentioned earlier. They are like incredibly dense bars about the dimensions of candy bars. I happen to like the taste of some of them, like the SOS bars, which have some coconut in them I think which gives them some flavor, here's a place you can get them:

http://www7.mailordercentral.com/longlif...1&mitem=180

Another thing that works, and comes in a few flavors are Millenium Bars:

http://www.countycomm.com/BAR.htm

Those are nice to have in the daypack.