Go to Valencia either during "Las Fallas" in March, the festival where they celebrate St. Joseph. The villages construct huge burnable effigies that represent the region or political floats. The floats then parade through the town and get judged. The best ones are sacrificed by burning. It's a pretty wild week of partying.
During Las Fallas, people throw huge firecrackers into crowds. I would be aware of one's surroundings. During the time I went a few years ago, it rained, so the festival wasn't as crowded.
See this website:
http://www.cyberspain.com/life/fallas.htm Another time to go to Valencia is in Aug.-Sept. during "La Tomatina". In Bunol, they cover the little tiny town in plastic so that thousands of people can have a huge tomato fight. Women should be careful when attending this event because it gets pretty wild and they will have their clothes torn off!
For La Tomatina, I wore swimming goggles and a bathing suit underneath my throw away tee shirt and shorts since you can't really hold on to anything during the tomato fight. It gets really, really slick. Those with tomato sensitivity should not attend. Imagine having tomato juice on your skin for 6 hours before you get to wash off on the outside showers! Do not wear any jewelry! Some of my girlfriends who were ill-prepared had their tops ripped off, but luckily, we were with classmates who were gentlemanly enough to offer their shirts to them.
See this website:
http://www.cyberspain.com/life/tomatina.htm Don't forget to eat homemade paella there! The best in the world! Stay away from the restaurants that cater to tourists where they serve frozen/"ready made" paella. One of my fondest memories is sitting at an outside cafe and seeing the owners of the restaurant walk back from across the street where they have been cooking paella in a 6 by 8 foot paella pan that needed four people to carry.
Enjoy!