First of course you can visit the historic downtown of Koblenz. Walk to the Deutsches Eck, take a look on castle Ehrenbreitstein, visit the churches and the fountains in the pedestrian areas. Also you can go shopping on the Löhrstraße, the main shopping street. Also you can visit the Löhr Center, a mall with 130 shops and located right next to Löhrstraße.
The next hundred kilometers south of Koblenz along the Rhine, you can visit several beautiful castles. For example the castle Stolzenfels (5 km south) or the castle Marksburg (10 km south). If you love castles, you have a lot of choices. The most beautiful one may be castle Elz (20 miles outside Koblenz). Or you can visit castle Bürresheim which you see in the movie Indiana Jones and the Last Crusade as castle Brunwald.
To the west, you can enjoy the Moselle valley. If you take the train to Trier (90 minutes and 40 Euro for round trip ticket). Take a seat with window on the left side in driving direction as the train has most times the moselle on the left side and you see more. Trier itself is the oldest city in Germany with around 2000 years. Visit the roman ruins, the porta nigra, the cathedral and the throne sale of the roman emperor, the Basilica of Constantine.
To the north, 100 km away is the city of cologne (Köln). You can go there by train in about an hour (40 Euro for round trip ticket). Visit one of the largest cathedrals, the Kölner Dom. Next to it, you can visit the Hohe Straße, the famous shopping street in Köln. Walk through the historic downtown and go in one of the typical bars there.
Closer places to visit are the city of Andernach (25 km to north) with it's historical downtown, the castle, the Rhine and the famous cold water geyser. From Andernach you can easily get to Maria Laach, a Benedictine monastery in a valley with a the lake in a nature reserve. Visit the over 900 year old church and take a walk to the lake. The 8 square miles large area is good for hiking with it's big woods and nice viewpoints.
Finally we have another thing you can do: Rent a car. Go on the highway and drive to the A3. German highways are often without speed limit and you can get a speed ticket if you drive too slow or slow down other cars. Minimum speed is 40 mph and suggested speed is 80 mph. And if you like and there is no speed limit, you can drive 200 mph. Be aware, that there are speed limits and you have to pay speed tickets. 20 mph too fast can cost you 100 Euro on a highway. For more information, watch here and here on youtube.
Read also: Call for Speakers, Hotels in Koblenz, Reasons to visit Germany and REAL Studio Conference 2010.