Light-weight GPS tracking

Hello. Can anyone suggest a good GPS receiver that's optimised for simply tracking routes and not much else? (ie. No display required.) I'm thinking of something light-weight that I could maybe stash on top of my pack and almost forget about, then extract something like a GPX file from it afterwards. I don't have much experience with GPS receivers but my understanding is that they chew through batteries. I suppose that part of the optimisation might be that it didn't wake up to check its location more frequently than necessary for it to survive a couple of days. Does such a device exist? I'm quite keen to be able to track where I've been, but so far I'm just trying to get an idea of what's available. Thanks for any help with this. Mike.