I had a really great time in 2012 when I visited the Spartan Races. This would be a really great photo opp for everyone (check out my Spartan race set on flickr).
There is a $10 parking and $25 entry fee cash only or $20 online. If paying online, just click on the registration button and select spectator option.
They do require a spectator waiver be completed upon entry, which can be downloaded from the site.