2. IAH is a good ways north of downtown Houston, and AFAIK there are no good and easy transportation options to take you anywhere from IAH for a few hours. There is a public bus service, but I'm not sure I'd recommend it. Certainly nothing like in AMS.

And besides, honestly, I don't think it would be worth the effort to visit Houston over a 5-6 hour layover. Unless there's a specific restaurant you're dying to visit, or you have friends to visit in town.

3. I'm not sure if you can access CO's Presidents Clubs coming off a BusinessFirst international flight, but if you can, head to the PC in Terminal E, the biggest and best one. (an onwards domestic first class ticket will not get you into the PC)

You can also purchase a daypass for $45, which could be worth it for such a long layover.

For the best food at IAH, Visit Papadeaux in Terminal E ^