Some empty hangers in a closet and an empty drawer or two are nice for stays of more than a day or two. Towels, washcloth, soap and shampoo in the bathroom. A box of tissues and an alarm clock in the bedroom. A coaster for the bedside table. If they've got a baby, point them to the location of extra cleaning supplies and cleaning cloths so they can tidy up messes without bugging you. Let them know where to dispose of dirty diapers.
If they're visiting from the US their electric stuff won't be compatible with the sockets or the voltage/current. A plug adaptor on hand for items with a AC/DC converter (laptop, digital camera) would be handy to have on hand, and if you have a spare hair dryer or similar things lying around letting them know they can borrow it would be nice.
Letting them know what they can help themselves to - juice and pop in the fridge, snacks in the cupboard, where the iron and laundry facilities are and how to use them if it's not obvious.
A quick babyproofing of the guest bedroom if the kid is crawling would be appreciated, I'm sure.
Maps and brochures for local sights.
For the not necessary but nice to have things which you can do if easy, but don't have to - a pitcher of water and some glasses, a couple of books or a radio in the bedroom, extra toiletries or OTC medications.
Finally, I would say don't be too over the top. If they sense that you're uncomfortable, or desperately running around trying to force more comforts on them, it may make them feel awkward. Set up the room comfortable, and be sure they know they can ask you if they need anything.