Make sure you see the Fortress Ehrenbreitstein, and Duetsches Eck.
Fondest memory: My best memory of Koblenz was trying to find a way into Fortress Ehrenbreitstein. We drove like an 1.5 hours to see this castle and it was closed. So instead of give up in defeat, we took a risk and tried to get in anyways. We ended up being on the wrong side jumping fences and stuff trying to get in, until we were totally blocked. So we turned around in defeat, and started to cross the river when we noticed little people looking over the edges of this castle. It was open!! so we walked to the entrance, which was an uphill battle the entire way to the castle. We were fools jumping fences to a castle already open! And I could have drove my car up to the castle parking lot, but instead we walked and jogged! What a workout