Do you have to stay at the Hilton Americas DT? It is a fine hotel, and there is more to do downtown than most here are alluding too but someplace like the Hotel ZaZa in the heart of the museum district might be more convenient and you can walk/use rail into DT (they also have hotel shuttles-with big longhorns on the front it is not cheap though). Just throwing that out there but maybe you are on points or prepaid at Hilton.

You mentioned you are "older people" so I am not sure what demographic, if you will, you are in when it comes to "fun". Do you want to do any shopping or just see the sights and eat great food? Mainly art or historical things also? You can certainly fill two days in the Museum District with the MFAH, Natural Science museum, Holocaust museum, Contemporary arts, health museum and a few other galleries. The Univesity of St Thomas has the Rothko Chapel and Rice Univesity has a couple of galleries. I could suggest more with a bit of a timeline from you; I did a fair amount of tours for foreign visitors to the city during my undergrad days on a summer internship, albeit a long time ago...and I still enjoy helping visitors^

BTW, The Menil is slightly outside of the main core of the museum district and not really on the rail line (yet).

Houston does have a double decker bus tourist company (around DT and the musuem area), and the greyline tours which can go out to NASA and even Galveston depending on which they are running. These could be options also.
http://www.houstoncitytours.us/
http://www.houstonchronicle.com/life...4825477.php#/0

http://downtownhouston.org/

It doesn't seem like you will get down to the Space Center, and every big city has a Zoo but there is also the Houston city pass which gets you into several of the museums at one price:
http://www.citypass.com/houston?mv_s...n=none&mkwid=e

If you are not going to make at least three of those it will not be worth it, and depending on your age you might get discounts from the musuems themselves.

Have fun, and eat well!