Hi Tindo,
In order to find accomodation in Valencia or any other city in Spain I use to check two different sites www.booking.com and www.muchoviaje.com being the first one of the biggest international reservation sites and the second the strongest national one you should be able to get the best prices.
About Valencia, you could spend two days visiting the city, the gothic quartier "Barrio del Carmen" is a jewel of the late medieval times in Europe, and the CAC "City of Arts and Science" is maybe the best modern architectural complex in the world. Other great areas are the Gran Via, "poshest" area of the city, Bull fight arena and the Docks.
The city offers too great opportunities to eat and go out.
After two or three days there will be little that you haven't seen in the city. You could choose to make a trip to some town in the region or visit one of the big cities of Spain. Within one hour of the city you can find Sagunto (old roman town with the biggest castle in Europe), Xativa (really nice medieval town) and Teruel (best example of mudejar architecture in Spain). Otherwise you could go to Barcelona or Madrid both around 350 Km away from Valencia.
Enjoy your trip!
Sebas