I personally bought a couple of Buffalo LinkStation Pros for this purpose couple with CrashPlan. I installed the NAS at my parents house and another friends house, created a mapped network drive on their computer to the NAS, then installed CrashPlan.
I setup CrashPlan to backup their important documents onto their NAS as well as the off-site locations. I also then installed another LinkStation Pro at my place to do the same thing (so each of us is backing up our documents to 3 separate NAS (2 off-site, 1 on-site). I've had this setup running for a few years now, and no issues to date. Restoring was easy when needed, I've had plenty of room on the devices for now (so long as they remember to NOT backup their DVDs that they copied to take with them on trips).
Downsides to this setup:
1) They need to keep their computer on so the mapped drive is available to CrashPlan
2) If they go on trips and only have a laptop, that off-site location will be unavailable for backup until they return.
However, CrashPlan will catch up the location when it becomes available as a backup location again, so the downsides are really small.