Stephen:
I've had good luck using a combination of the following:
1. State maps (folding ones, that you pick up at the welcome center on the interstate). Free and usually have scenic routes marked. I always ask if there are any cycling specific literature when I stop at these as well.
3. State DOT. These are often free and online. As examples:
Kansas,
Texas, and
Michigan. A simple <insert state name here> bicycling routes generally turns up something. Wisconsin has a "
Rustic Road Designation", these can be good road for cycling
4. Google "scenic drives in <state>". These are often targeted at automobiles or motorcycles, but no reason why they would not be scenic for cyclists as well.
5. I've seen good cycling routes in some tourism sites as well. In our state, see this
TX Highways article6. Look up the outfits that do guided and supported cycling tours (Adventure Cycling, Trek Travel, Butterfield & Robinson). See where they do their tours. Often times they will have daily itineraries and you can connect the dots. If someone is charging top dollar for a tour, you can be pretty sure that it is scenic and a nice place to ride. I think I did this while planning a multi-day ride in the TX hill country.
7. Books. Invariably there is a book with routes. In TX, I've seen "The best bike rides in Texas", and "Cycling Texas". These are usually quite old, but scenery does not change much and the books can be had used fairly cheaply.
8. Delorme Gazetteer. The smaller and more squiggly the road, the better.
Like you, I also live in Dallas. We could use more scenic routes.