I live further north (The Woodlands), so don't have many route ideas. I used to live over there a couple years ago, but I wasn't doing much cycling then. I know there's a marked bike lane on FM 529, and I think Barker Cypress has some nice shoulders, IIRC. It's gotten pretty high-traffic in the last few years, though. There may be a way to take 529 all the way out to Katy and hook up with some of the Katy Flatland route--I rode that charity ride this weekend, and there are some nice roads out that way.
When I did a ride with my church group for a while this spring, we left from the Target at 290 & Spring Cypress, went out the back way to Cypress-Rosehill, took that to FM2920, took a right and hit Telge, took that to Grant, then took Grant back to Cypress-Rosehill back to the Target. That route was about 20 miles. It was low traffic most of the time, but 2920 is the only road on that route with a good shoulder/bike lane and we were only on it for a mile or so. You could possibly extend the loop either direction on 2920, but I'm not terribly familiar with it west of Tomball. I know it goes all the way to 290, but I'm not sure what the cycling conditions would be, or if there would be any loopable roads that direction. If you go east through Tomball, then you're getting into my stomping grounds.
You might try contacting Northwest Cycling club for routes in that area. I hear their Saturday ride is excellent. You could also try stopping by Northwest Cyclery at 290 & Jones. They're the primary sponsor for the club, and would probably have someone there who could give you some direction.
Hope this helps!