Ok then, some suggestions, as I've been to Valencia a couple of times.
Just north of Valencia is Requena, a fantastic little Spanish town. The night before La Tomatina is the festival of water and wine, but as you're going in December, you can just enjoy the town itself. It used to be a Moorish fortress, and parts of the town date from the 13th century. All sorts of random little streets and twists and turns.
Just north of that again is Buñol, where La Tomatina occurs. Again, nice town to walk around and explore, cool squares to sit with a coffee and soak it up. I spent several hours walking around here trying to find anyone who would let us use a phone, after being pickpocketed at La Tomatina, so I know it well. At least if you go in December there won't be passed-out Aussies everywhere ;)
90km north of Valencia are some hot springs. I wanted to go, but found it really difficult to get information on them, and didn't have much time anyway. But they sounded incredible.
Finally, and I know this is vague and really unlike me as normally place names are easy, but there's a Unesco heritage site - a castle fort village place on a hill. It's SPECTACULAR. But for the life of me I can't remember the name. It's 2 hours by car from Valencia. My searching has revealed Xàtiva which seems to pretty much match it and looks amazing, but the images don't jog any memories currently. Still, it looks incredible too :)
Ok, after finding this blog I'm now a little more convinced that it may well be the right castle.
Also, and I know you didn't request it, but in Valencia, go see the Holy Grail, and the new area of Valencia is spectacular and well worth a walk. You can also go down to the docks and see the America's Cup Village with all the companies that compete - Team New Zealand, Oracle and the like.