I second suggestions of Sankeien and Nikko. Hakone is another popular destination that's not far from Yokohama (I'm pretty sure it's significantly closer than Nikko). I've not gone as a day trip but it should be doable if you plan well. And because it's in the mountains, the leaves *might* have started changing by then. It's also famous for onsen (hot springs).

Also, I'm not sure which Intercontinental you're staying at (Yokohama now has two) but both are quite central. If you like nightlife areas, I recommend checking out the Noge ("no-geh" 野毛) area behind Sakuragicho Station. It goes on for blocks and blocks and blocks, full of pubs and restaurants and such. The building known as Miyakobashi Shotengai (都橋商店街) is particularly cool. It brings back memories of the postwar Showa era.

Enjoy my city!