The railway options will not take you anywhere special when you are restricted to a few hours ride. You will be far better off to see some nice scenery if you hire a car, and drive along the Oslo fiord.
The bus ride for Drøbak is inland, but quick and convenient. If you feel you can use the time and the weather is nice, there is an option on going there by a ferry - having some beautiful scenery on your way - and return by bus (the ferry mostly runs only one return journey daily, and will not allow you enough time to see the town if you opt for a return trip).
You may check it out on the visitoslo.com site.
If you feel you can use enough time it is possible to go to Hamar and Lillehammer (1994 Olympic resort) by train. They are both located on the shores of Lake Mjosa (Norway's largest lake). Lillehammer has also got a stunning outdoor museumof old farms and houses (Maihaugen) - which in many ways is similar to what you will find at the Folkemuseum on Bygdoy peninsula in Oslo. Bygdoy also houses the viking ships museum, Thor Heyerdahl's Kon Tiki-museum, The Norwegian Naval museum and the Fram museum (Fram was the vessel used by Frithjof Nansen and Roald Amundsen for trying to reach the North Pole).
Enjoy your stay in Oslo, Tinsie !