First of all, you can visit many attractions. Tokyo Disney is currently the most popular attraction among Asian tourists, and I d strongly recommend it since it is a lot of fun! I also recommend Sensoji Temple, Imperial Palace, and Ginza for sightseeing, also Roppongi and Shinjuku for evening/nightlife.
I can’t give you the recommendations without knowing who you are and/or what you guys would like to see or do though. You may want to consider taking a day trip to Kamakura or Hakone/Mt Fuji. Kamakura is an old town with many temples and shrines, and you can see cultural Japan there. It is about an hour away from Tokyo by JR train. Kamakura is sometimes called the Kyoto of Eastern Japan.
It is highly recommended if you don’t visit Kyoto. Hakone is part of the Fuji-Hakone-Izu National Park, less than 100 kilometers from Tokyo. You can take a train or bus from Shinjuku/Tokyo/Shinagawa/etc to go to Hakone. You may be able to enjoy seeing “Mt. Fuji” in Hakone if the weather is nice.
The Oriental Bazaar is one of the best “one stop shops” in Tokyo for tourists, it has four floors of shopping stocking everything from kamikaze T-shirts and headbands to large-sized kimonos, there are also English books on Japanese design and culture. One of the best places for your holiday shopping The Bazaar is housed in an instantly recognizable building with a faux-Chinese temple roof.
Be aware the Bazaar is closed on Thursdays. To find the Bazaar goto Omotesando station and walk down the main street towards Harajuku, the shop is on the right about halfway down the road.
Exchange money in advance. It will be inconvenient to locate banks to exchange money while you are at Japan. The hotel will of course, give you poor rates. ATM’s are workable but no as easy to find as other places. 17 or 18 in the morning to mid-20’s in the afternoon. Lots to do in Tokyo. Meiji Shrine is a possibility. Asakusa for the temple, Ueno for a nice park, Shinjuku for walking around and general feel of what Tokyo is about.
The voltage is not 220V. It is 100V. Most digital electronics (cameras, computers, cell phone chargers) can handle the range from 100V to 240V. Just about all Japanese hotels have hairdryers. Note that your phone in Singapore will not work in Japan unless it’s 3G. Also, the Japanese electrical plug is two flat pins, similar to America. For a couple dollars, you should be able to buy a plug converter in advance.
Taxis in Tokyo are very expensive. Also, traffic is usually bad to downright impossible during many hours of the day. You should take trains or subways instead to get around places cheaply and quickly. There is no question that you will get lost here and there, but just have a good map and be patient.
Japanese are usually very willing to help even though they might not have best English command overall. I highly suggest that you invest in a guidebook, one that has good and concise information but not too thick.