I'd say a laptop would be the best option - they're less intimidating than the big box churning away in the corner, and they can set up anywhere - on the kitchen table, the couch etc.
A netbook would be the cheapest option, and it's small and light enough for them to have in the living room and pull it out when needed. The poblem might be the small screen and their eyesight.
A Macbook would be best. They've an easy to use interface, require next to no maintenance and you don't have to worry about anti-virus or such like, so there would be no worrying messages popping up for them. There's also a very good software suite (iLife) included. The problem is the price. You can get a computer that would be good enough for a lot less. For lack of headaches, I'd go for a Macbook, but it is your money.
Whatever you get them, make sure you do a backup of the system when you get it the way you want it.