You're right - Kiel is pretty blah. It was largely destroyed during WWII and there is not much of an attractive streetscape. Instead I would head to Lübeck, which is a really pretty city and close to Kiel. The Baltic cities are all in the style called 'brick Gothic'. Before you go, read something by Thomas Mann, who is a major writer who lived there ('Buddenbrooks' is the classic but it's quite a hefty read). Günter Grass, another famous writer, is also honoured with a museum there. The Holstentor is the remains of a 14th century gate, the symbol of the town, but now also a museum. Visit St Peter's Church, and go up un the lift to the viewing platform, to get a great view of the whole city. Have coffee and cake in Café Niederegger, which has been in the same family for 200 years. Its speciality is marzipan, which might come as a layer on your cake, or you can also order marzipan coffee. And the cakes are a big deal, a really amazing display. You can easily spend a pleasant day there.
http://www.luebeck-tourism.de/discover.html

Slightly further away is Hamburg. A different experience to Lübeck because it is a big, cosmopolitan city. I enjoy Hamburg, and its drawcard is the parks and how it is built on the water (the Alster is the river). The food is distinctly north German (think smoked eel). A famous drink from the city is an Alsterwasser (kind of a beer shandy, with local variations). You can do a tour of the harbour, which is a working port. It is also a very elegant city, with good shopping and restaurants.
http://english.hamburg.de/tourist-information/

Both options would give you enough to do for a whole day. Both are just over an hour to get there by train. I would probably pick Lübeck over Hamburg because it's just a tad closer, but both are interesting and deserving of a visit.

Lavandula