If you're looking for closed-course riding, then Berthoud, Milliken (Two Rivers), and Watkins will be your best options. All require memberships, but Berthoud and Milliken will let you buy a day pass. Not sure if Watkins will do that, but it's going to be the next closest track to the Lakewood area.
I wouldn't recommend Erie to any visitor whom I wanted to leave Colorado with a good impression of our MX tracks. Berthoud is by far the best quality/prepped track that we have to offer in the Denver area. Lakewood, Watkins, and Milliken are all pretty comparable with one another from that perspective.
I definitely wouldn't want anyone to just see Lakewood and think that's the best MX riding we have in Colorado. It absolutely is not, IMHO.
If you're looking for trails, there are plenty of those too. The terrain is generally dry, loose, rocky, moderately forested, and usually provides significant elevation changes. Rampart Range will keep you busy for a couple of days, easily.
Bring a bag of jets with you. And if you need directions to the MX tracks, just holler..