South Table Mountain is another work in progress, although it is open. It is another joint effort between the City of Golden, who owns the "slopes", and Jefferson County Open Space, who owns the mesa plateau. The riding is generally easy in difficulty, both technically and aerobically, so it makes for a great early season ride. Beginners can also feel comfortable here.
There is also a zeolite rock quarry on the east side, a little north. This provides a little cliffed out area, and a trail rings the cliff-face.
More info on JCOS' website
Trails are either nice singletrack or old road, which is basically parallel singletrack. The majority of the track is sand and gravel, and occasionally clay. Once you've done the principle gain, the mesa doesn't provide too much in the way of gain/descent, although there are a few small climbs on the NE side of the mesa.
Jefferson County Open Space is getting ready for some new significant changes at the park, including a new parking area (since there isn't one now), as well as other trail modifications and more. See the link for full details.
There aren't really any large parking "lots", but access can be had at several locations.
19th St. dead ends at a City of Golden "TH", where there is room for maybe 3 cars. This provides the biggest climb of the whole park.
There is streetside parking on the south side of STM - take South Golden Road east to Quaker St, head north to Golden Hills Road, which parallels the south side of STM. The streetside parking is evident.
There is also 2 parking areas on the east side behind NREL - one is more on the SSE side, and the other is on the NE of the east side, by Rolling Hills Country Club. Both of these provide a nice climb, and a nice descent at the end of the day. This is the side that JCOS has spent more time developing.
When to Bike
Like almost everything in JeffCo, mid-day rides in the middle of summer can be brutally hot. The only time that riding isn't recommended is after heavy rain/snow, when (like everywhere else) trails can be muddy.
STM is so big that even if every bit of parking was taken, you still probably won't see too many people up there. It is a big space.