Your favorite city near Prague is? (can be outside CZE)

Plzen.

Brno.

L'viv But then the list provided so far is pretty much "towns" as opposed to Cities.

Berlin

Cesky Krumlov is a very popular CZE city, and is a couple of hours south of Prague. There are a number of mini-bus companies that take travelers on excursions between Prague, Cesky Krumlov, Salzburg and Vienna every day of the week for very reasonable fares.
They could also drop you off over to Vienna where you could catch the train or ferry boat to Bratislava (39 miles).

Krakow. I've heard it described as the next Prague. Since you like history....Auschwitz is near Krakow. Visiting was a very humbling experience.

Karlovy Vary is a spa town also known for it's chandeliers and garnet jewelry shopping. Not a real big town, but if you do the spas, that'll fill up your days.

The OP is a "young adult" who's "not really looking for a quiet small town". Karlovy Vary and Cesky Krumlov are exactly what he's not looking for.

Val, I'd also suggest looking at Cesky Krumlov. There are lots of Bus companies in Prague offering day tours. While it would be better to spend one night there, a day trip will provide a quick look. Happy travels!

Thanks you so much for all the replies! By all means, I am not looking for a party city or anything...I am just very active and want to keep myself busy...in a city! I will look into the smaller cities that you all mentioned or town as they could fill one or 2 days up. And, I have been to Krakow (wonderful city I agree)...but should have listed it in my message. Martin & James...although obvious...I have never been to Berlin!
So, I will check if I can find a good flight from Scandinavia. Martin, good job on reading my post carefully :)

Vienna. CK is a daytrip with one night.

I agree with George. The Czech Disneyland isn't what I would recommend to a young guy (who doesn't want to see a small town anyway).
Cesky Krumlov is more than 3 hours away from Prague. How about Dresden instead, which is only 2h away? The city is a good mix of old and new, has great museums, the Neustadt area is lively and fun, and the surroundings offer everything from small towns to castles, vineyards and mountains.

Another vote for Dresden. And if you are active and like biking or outdoor activities, renting a bike along the Elbe River east of Dresden is a nice option for a daytrip. The train ride between Prague and Dresden is pretty awesome too. Berlin of course, but I don't consider that near Prague. It's about a 6 hour train ride.

@George
That's one of the reasons. The other is that this sugar-sweet Czech town that tourists see today never existed. Historically it was a German town with a Czech minority that saw the ethnic tensions and violence of the 19th and 20th century, which culminated in the German invasion, the expulsion of most inhabitants of C. Krumlov after 1945, and the loss of local identity and culture. Try to find this history today.

In anglophone historiography look at "Nemesis at Potsdam"...a bit dated, not the best, but still serves as an introduction.

All cities in the Czech Republic (apart from Prague) have far less inhabitants than 1 million. There are nice places to visit, but from what you wrote I would suggest to take a chance and do a trip from Prague to Berlin via Dresden. Dresden is one of the smaller EU cities (I believe around 1/2 mil. inhabitants), but it is a nice break on the way to Berlin. Berlin has great history, good nightlife and is near Prague. Whether you go by private car, bus or train, it is definitely better than Karlovy Vary or Cesky Krumlov. In regards to Karlovy Vary; I like the town for afternoon to walk around and then get back to Prague. The city is boring once you stay more days and you should avoid staying over night, unless you wish to do the spa treatments. Cesky Krumlov is one of my favourite towns in the CR, but I prefer not to travel there alone. If you would prefer flying, I suggest Dusseldorf in Germany as another option.

My vote would be for Berlin or Munich - both quite vibrant and relatively easy (and cheap) to get to from Prague.

Lednice-a small village in south Moravia. palace and gardens on the way from/to Bratslavia

Olomouc, the 2nd largest city in CZ. Good train connections from Praha.

Actually Olomouc is the 6th largest city in CZ, after Prague, Brno, Ostrava, Plzen, Liberec, nevertheless I agree it's worth to visit. You can find information about it in Rick Steves book: Prague and the Czech Republic.