Stockholm is definitely worth a visit. Guidebooks might be a good way to see what else there is in Sweden.

http://www.flickr.com/photos/mksfca/sets/72157623071349528/

Are you on your own or with someone? How old are you?

If fit and with a friend then consider building your own raft and rafting down a Swedish river.
http://www.visitsweden.com/sweden/Attractions/Outdoor-activities/Water-activites/Timberrafting/

My son and his GF did this a few years ago and loved it. Harder work than they had expected, but definitely one of there best holidays, and a unique way to see the countryside and the wildlife.

Try Google or get a travel guide. There are so many unique things about Sweden, but it depends on what appeals to you. It is like asking what is to see in California.

Sweden is amazing - we loved Stockholm.

How about a rooftop tour?
http://www.visitstockholm.com/en/to-do/attractions/roof-top-hiking/974

How about a visit to the Ice Hotel? (when are you going?)
http://www.icehotel.com/

Go sailing!
http://www.stockholmadventures.com/Sail

just come back and report....

Adventurous 4 young adults. 7 day trip so which cities do you guys recommend besides Stockholm? And also hotels, authentic restaurants, and transportation. We are planning next year, January or February of 2012

-Thanks guys

I'm not the expert, have only been to Stockholm, but loved the city, the surrounding water (Stockholm consists of 14 islands) the food, and the Vasa was fascinating to me.

If you eat meat, be sure to have reindeer, delicious, and pick up a tompte to take home with you. I have several, and one guards my back door.

http://tomtar.se/wordpress/

There is a covered market also. I missed that somehow, can't believe it, but I've seen it on No Reservations and Bizzare Foods and it looks fabulous. I won't miss it again if I ever get back to Stockholm.

Old Town, Gamla Stan, is a neat place to spend some time, we stayed there, just around the corner from the Palace and the Stockholm Cathedral which are both worth a visit. Also the Nobel Museum, he was quite an amazing fellow.

You can take a boat ride or a ferry to some of the islands. It's really a fun city to spend some time in.

January or February of 2012 will be short daylight, cold temperatures, maybe snow.

If you search here, you will find people recommend a visit to Uppsala, but I have not been there; only Stockholm. There are not many cities with tourist destinations in Sweden.

Cross-country skiing?

A friend of mine living in Sweden took the train to Trondheim in Norway (long trip) and said it was one of the most beautiful routes he has seen. I'm not sure how far you have to go to get to the scenic part.

Janaury or February, some are recommending August or September. ummm.. Snow don't want to see snow when we go there. If I want snow, I can just go to Lake Tahoe. lol How about cultural places in Stockholm? Like museums, arts, history?

Click on "view by country"at the top of this forum. Choose "Sweden". In the "Sweden" sub-section, you will find dozens of trip reports on Sweden and Stockholm which you might find useful.

If you are young adults and looking to have fun, Uppsala is about 1 1/2 north of Stockholm via train and is an old midevil town that is also home to one of the largest universities so there are a lot of young people going out at night. Another similar town would be in the southern part of Sweden called Lund. It's about a 30 minute train ride from Malmo and an hour train ride from Copenhagen. Everywhere in Sweden is safe so you don't need to worry about safety. Have fun.

Asking a general question such as what to do in Sweden, won't give you the answers you are perhaps expecting.
Why don't you do a research by googling Sweden or taking a look at some tourist guides and then come back to the forum to ask for more detailed informations ?