I didn't even realize that ds and I will be in Tokyo for the lunar new year. Should be a lot of fun!
The Yokohama area is quite removed from Tokyo -- I would not recommend staying there. I'd recommend Ginza, which is walkable, and has several great shops and restaurants or maybe Shinjuku, which also has several nice restaurants, but it is more of a business district. They might find Shibuya too crowded for their taste, but it is an excellent location.
I found the Marriott hotels in and around Tokyo to be way too many points versus cash cost. I transferred URs to Hyatt and booked ds and I each rooms at the Hyatt House Shibuya (rooms have washing machines, which will be nice). I'm also eyeing the Hyatt Centric Ginza, but it would cost me more points and I don't think we will be in our rooms too much. I don't find value in using MRs for hotels (I use them to transfer to airlines) and with the relatively low cash cost for hotels in Tokyo I would try to avoid using them for a hotel stay. There are also a lot of local hotels in Tokyo that are priced much lower than the NA brands which are highly rated.
They really should take some tours while they are there -- there is so much to see and do. Or they could hire a private guide to take them around, but honestly transit in Tokyo makes getting anywhere super easy and announcements are in Japanese and English.