Short Scandinavia Tour - 3 Days:

With 3 days to spend on your Scandinavia itinerary, visit Denmark's capital Copenhagen and Scandinavia's popular south. Copenhagen offers great shopping as well as relaxation in the beautiful Danish Royal Gardens.

During your stay, take a day to visit Sweden, which is a short trip away from Copenhagen (across the Oresund Bridge which connects Denmark and Sweden).

Recommended reading:
Destination Copenhagen: A Travel Guide
Things to Do in Stockholm
Train Travel in Scandinavia
The Oresund Bridge

Medium Scandinavia Tour - Staying 6 Days:

If you have about a week for your itinerary, take the step above and add Oslo (Norway) to your itinerary. You can either rent a car to drive there, or still use the train system ScanRail to your Norwegian destination. The capital offers many attractions, among them Oslo's Wonderful Parks.

Recommended reading:
Destination Oslo - A Travel Guide
Shopping In Oslo

Long Scandinavia Tour - Staying 9 Days:

With 9 or 10 days, follow the steps above, plus the "Norway in a Nutshell tour." This 24-hour tour consists of well-organized ferry, bus, and trains in Scandinavia. It shows travelers the famous fjords and the towns Flam and Bergen, known to be a great place to see Scandinavia's Natural Phenomena. Or, take a day off from your itinerary and enjoy some city sightseeing!

Recommended reading:
Scandinavia's 3 Natural Phenomena
About Norway

Long Scandinavia Tour - Staying 12 Days:

With a 12 day vacation, use the itinerary steps above, and just add Finland to your schedule!

Add Helsinki, capital of Finland to the end of the itinerary described above. The ship takes 14 hours to reach the city: this comes in handy if you choose a departure time at night, and sleep during the trip to Finland. Wake up nicely refreshed in Helsinki!

Recommended reading:
Countries Of Scandinavia

Extra Long Scandinavia Tour - Staying 16 Days:

If you have 2 weeks or a little bit more, I would suggest to finish the steps we've described and enjoy nature and local culture by visiting the Danish towns Ærø (Aero), Odense, Frederiksborg, and Roskilde. Roskilde has great music events and cultural events, and Frederiksborg offers natural beauty in its Royal Gardens.

Recommended reading:
Denmark's Royal Gardens
About Denmark

Extra Long Scandinavia Tour - Staying 20 Days:

With this long a Scandinavian vacation, you are to be envied! If you are lucky to be able to enjoy Scandinavia for 3 weeks, use the itinerary we've created so far, and then go see Jutland (Denmark's peninsula), e.g. the amusement park Legoland in Billund. Another point of interest you may want to include at that point would be the Swedish town Kalmar, by the Baltic Sea. When you're there, make sure to see 12th century Kalmar Castle which has played a crucial part in Swedish history.

Recommended reading:
Legoland in Billund
About Sweden

Practical traffic tips and driving advice can be found in the useful article Driving in Scandinavia.