Want to see as much of Oslo as possible in a short time? All things here are in walking distance if nothing else is stated. Here is one itinerary if you like museums and want to learn a lot during your trip in Oslo:
Go from the Main square to the Royal Castle, seeing the parlament and all the important buildings and turn left into Drammensveien. Then walk past the American Embassy (the most secure building in Oslo) and then turn right on a road called "Frogerveien" and you will find yourself in Vigelandsparken, which is of Oslo's most famous parks. . Take the Metro back to Stortinget and head out to Bygdøy to see, "Villa Grande" (The house of the Norwegian Nazi colaborator Quisling) and then the Norwegian Museum og Cultural History which is located 10minutes walk from Villa Grande. The folk museum contains alot of preserved buildings. Then walk down to the Fram Museum,Kon-tiki Museum and the Norwegian Maritime Museum before taking the ferry (or the buss nr30) back. The time now should be around 18.00 so you can go out and do some barhopping.
Now you can use day 2 to roam around in Akershus festning (Akershus Fortress) to see the 4museums there (i have reviews here on Travbuddy). We Norwegians are very proud of our ww2 occupation history , and its all documented in Akershus festning.
There are also great views of Oslo Harbour and the city hall from Akershus Festning. The cruiseships also dock right next to Akershus Festning.
After Akershus Festning you can get on the Metro to Holmenkollen to go up in one of the most famous skijumping hills in the world. Just follow the signs from the Metro Station. There are also GREAT views over the Entire Oslo from Holmenkollen, you can actually see Holmenkollen from Akershus Festning if you look a a little left of the city hall.
If you didnt find the museums in Akershus Festning interesting and its still early you can catch a ferry out to "Hovedøya" (leaves from Aker Brygge,next to the City hall)to see ruins from an old cloister built some 800years ago and also the old defensive structures located on the island. Norways first Airport where also located here,but now the Hangar its a storage room for private boats and nothing to see today.
If its getting late skip Hovedøya and go back to the main square and then up to Storgata (5min) for Oslo Reptile Park. Its not really worth the admission,but if you have an Oslo pass and have some time to kill this is a perfect choice.
For drinking? Preparty is very popular in Norway (Of voirspiel as they call it, A german word) so normally there isnt to many people at the nightclubs before 23-24. But many other people also go to a pub in the "Grønland" Area for cheap beer. Its 5min walk from the Main square. You have three cheap bars lying next to each other (Bob`s , Stargate and another bar that just changed name). On the same street you have a very cozy bar named Asylet (perfect place for a date or something, but its not so cheap as the neighboring bars) and Nightclub(s) like "Dattera til Hagen"
The BEST site for guide about nightlife is http://www.oslopubs.com/Oslopubs.asp
Please give me feedback about what you think about this itinerary