Valencia is a beatiful and no too big city, so you can see it in a few days and discover nice places.
If I were you, I´d walk around the city centre and I´d gaze at some of the monuments.
I suggest taking a walk along the district named “El Carmen”. It´s a nice and enjoyable area where you´ll find intesresting restaurants and pubs to have a drink.
Don´t miss visiting “La Lonja”, is one of the most beatiful medieval buildings in the city. Next to La Lonja, is the Central Market ( a nice example of modernist architecture together with the North Station).
You should try going to “Ciudad de las Artes y las Ciencias”, above all to see the amazing architecture, but visiting the facilities is a bit overpriced.
Last but not least, I find taking a tourist bus a bit dull, and when you go for a walk around the city centre or at metro, watch out for pickpockets.
Enjoy your holidays!.