I have a Garmin 60CS. Excellent unit and I'm sure you'll love the sucessor.
I use NiMh batteries in my unit and have almost exclusively since day 1. I use it about as frequenly as you say you will be. NiMh batteries really need to be stuck somewhere for a loooong time before they start leaking. The 60 CS, and I imagine the 60 CSX has a feature where it will take a reading off the altimeter (Basically a barometer) every 15 minutes, but you have to leave batteries in it for it to work.
So what I do is I have my trusty old MaHa charger. I have a small army of NiMh batteries and I just try to keep them charged up, and I usually really just keep a set on the charger always, since they'll just sit there and trickle charge at an infinitely small level.
When it's time to use the GPS I'll just swap whatever batteries are in it out for the ones in the charger. Once in awhile I'll forget and take it out with just the batteries that have been sitting in it. That usually makes things, uhh, interesting, but I've had the GPS for 4+ years, and I think only once have I been stuck with batteries so flat it wasn't really working right.
I also have some Kirkland AA batteries around the house for those things that rechargables just are not ideal for, and so when I am going to be gone for more than a couple days I will pack a set of those just in case, since my digital camera and countless other little toys all use AA batteries.
So I would just say go for rechargables. If you have some alkaline batteries on hand you should always be safe if you go to use it and find you have no charged batteries, and if you really want to be safe, you can keep a pack of the Lithium AA batteries in your car.
I usually know when I'm going to need it, so it's not hard to ensure I have charged batteries. The thing I used to use it a lot for, on short notice, was driving directions around town. Well I have a phone with maps built in now, and while it's not *nearly* as good as my Garmin, it's taken a lot of the load off the Garmin for light around-town navigation.