This info is going to be overkill, since you said you only have a couple of hours, but you never know, so here goes....
The November, 2009, issue of
Texas Highways has a couple of pages about things to do in the RGV. Here's a link to the tease, which lists seven of the 15:
Postcards: Things to see in the RGV
Others in the article include:
The Museum of South Texas History in Edinburg (
www.mosthistory.org);
The Valley Nature Center and Frontera Audubon in Weslaco (
www.fronteraaudubon.org);
The McAllen Art Walk (
www.mcallenartwalk.com);
The Resaca de la Palma State Park in Brownsville (
www.worldbirdingcenter.org/sites/brownsville;
The Harlingen Mural Project (
City of Harlingen Texas).
A couple of other things I found after reading the article were:
Little Graceland in Los Fresnos;
Iwo Jima Memorial and Museum in Harlingen;
Freddy Fender Memorial in San Benito.
I haven't been to the valley for years, and I've never seen any of these places, so I cannot vouch for any of them. Some of them are in the opposite direction too, but if you're a huge Freddy Fender fan....
Have a good trip, and let us know if you find anything.