Daniel,
For most people attending the NNGC, the clinics being offered in the morning and evening are one of the big draws. These consist of presentations about various historic narrow gauge railroads, as well as modeling information. The convention site itself will have a number of display layouts, a contest room with the entries displayed and a manufacturer's exhibit area. Generally the convention site is closed in the afternoon so people can visit various home layouts or other sites of interest. I've always found the home layout visits to be outstanding. Of course the museum in Golden and the Georgetown loop are less than an hour away.
If you are coming all the way from Europe, I would suggest that you also consider visiting the Cumbres & Toltec (Antonito is about 6 hours from Denver) and possibly the Durango & Silverton. The Cumbres and Toltec is offering two special events aimed at Convention attendees. The C-18 class 2-8-0 #315 will be pulling a passenger excursion with runbys on August 28, just prior to the convention and the Friends of the Cumbres & Toltec are offering 3 days of photo freights. The first day, September 4, is aimed at convention attendees and the next two days are for more serious photography, including a two engine train on September 5.
For informaiton click here. Of course, you can also ride and photograph the regular service trains as well.
Whatever you decide to do, I'm sure you will have a great time.
Michael Allen