I went to Magic Mountain yesterday and it is a surprisingly good place to take a date(other than the glaring need for alcohol).
Most theme parks are btw $ 30-40 a person and can be fun for 4-6 hours easy. So it's not WAY more expensive than going out for a few drinks in a city like LA, NY or Vegas.
Plus it's a lot of fun, gets the heart going, and gives PLENTY of opportunity for built in conversation. I definitely am going to start taking more dates there.