As of a week ago the internal (non-public, int'l inbound connections) TSA checkpoint for int'l arrivals is still open so no need to fully exit if staying at the airport. This is before the final CBP area exit just after the luggage recheck desks. The main public E checkpoint is closed for construction so upon return you can enter at D or any A/B/C and ride the elevated train to D, or take the pre-security subway to D...plenty of options.

As others have noted you have plenty of time to do things. If you figure an hour to pass CBP and get a rental car (or Uber, etc) and returning with two hours to spare you still have 8 hours. You could drive a lot of places, in that time you could go to the beach in Galveston or Lake Conroe or the Big Thicket, national forests LOL. That said, there are fun, quirky museums on the northside near the airport, even starting that early in the morning. Breakfast tacos and Kolaches are certainly worth a try that early :-)

https://www.nmfh.org/ The National Museum of Funeral History is worth a visit up on the northside. Sometimes the topic bothers folks but it is very interesting.
https://artcarmuseum.com/ The Houston Art Car museum is in the heights and comes from Houston huge Art Car Parade

https://houmuse.org/ The museum district has amazing facilities, with several world renowned and award winning museums with 19 museums total in four walkable areas (yes summer is hot...just like everywhere else in the country...'Tis why it is called summer, but yes that makes long walks at mid day harder LOL)
Natural Science has great exhibits, FIne Arts is great, the Holocaust Museum is a moving experience, and Menil just to name a few.

NASA is NASA and very cool- Space Center Houston is the visitor centre now and you can take trams to rocket park and see a real Saturn V rocket, go into the original Apollo mission control which was just refurbed into how it was for the moon landings, with ashtrays and all, plus you get to see the new mission control (there are two, one working the ISS, and the other would do the shuttle or now newer missions) and walk through the training area live as stuff is going on. The walkway is up top so you look down and across all the training.

San Jacinto Monument, Kemah Boardwark, the Galleria which is the biggest Galleria in America and a destination where you will find people from all over the world shopping. Shopping is shopping but it is certainly its own world and has the waterwall and great food nearby if you are in the mood for such things.

Houston is now the most diverse city in the country and with that comes food of all kinds, and amazing food at that; Texas brisket BBQ and Tex-Mex are certainly local favourites.

UofH and Rice U have various museums too; TSU, St Thomas, and HBU do also but they are smaller. Presuming you are solo, but with kids the zoo and Hermann park train, Japanese garden, spiral mountain, and statue garden are fun for kids, and near the museum district. THe Downtown Aquarium and Discovery green are fun for kids too.

If you like beer there are breweries like Karbach, St. Arnold, Buffalo Bayou, 8th Wonder and many more.

There is plenty more, just let us know what you are into and if you want to uber or rent a car. Car rental is usually cheap unless OTC or another convention is in town. Sadly the direct bus no longer runs and the 102 takes forever; the train is not connected to the airports yet (thanks Taxi mafia...)