you could easily split between Tokyo and Yokohama. IC Yokohama Grand was a longish walk for your parents from the tube 600m?, but they would love the views and 3-4 days could easily be spent there. I was there in may as a platinum and very happy, like you the price drove me there at 1/2 to 1/3 the Tokyo prices. Tokyo is about 40mins away for 700ish Yen.

I Spent a few very happy days at Tokyo Bay in May in the 52m Junior suite, the views of the river and harbour were fantastic. pretty expensive at $500USD ish per night but worth the splurge. The tube is very close 300m and there is a pier about 100m away for Harbour cruises. They also have a shuttle that takes you to the train station for the Sinkansen trains