There's no doubt that south Texas would be marvelous! I wanna go, too!!
But there are other marvelous places, some very seasonal, that truly are hotspots.
And not that I've visited them, but jeez -- I read a lot, and I can dream, can't I?
Cape May, NJ -- one of the great grand-daddy of hotspots on the continent. Warblers, raptors, shorebirds, waterfowl. Spring and fall migration. Atlantic shore and the Delaware Bay (and just 2 hours from Atlantic City if you're a gambler).
Hawk Mountain, PA -- the mega-grand-daddy of hawk watches, the very first. Incredible raptor watching from September through November. (OK, I've been here -- and getting there, you can claim to have been on the Appalachian Trail, which traverves the same ridgeline.)
Both of these places are easily reached from Philadelphia.
Crane Creek SP, Ottawa NWR, and Magee Marsh, all in NW Ohio. Good any time of year for a variety of things, but MAY is prime time warblering. Crane Creek and Magee (which surrounds Crane Creek, the SP is right on the Lake Erie shoreline) is where the warblers stop and fuel up for that 'pelagic' trip to Point Pelee. The boardwalk through the wet woods at CC is generally butt-to-belly with birders in May, and for good reason. A stop here on a day trip several years ago gave me 26 lifers in about 2 1/2 hours.
All this area was formerly the Great Black Swamp -- all drained and flat (and pretty unattractive) now, but the birds still come where the wetlands are being preserved.
AND, there's Cedar Point. If you're into amusement parks, need I say more?
Easily reached from Toledo, Detroit or Cleveland.
Salton Sea -- don't miss it.
Easily reached from San Diego -- and San Diego County has been called the 'birdiest' county on the continent.
And if you go here, for god's sake, take the time and spend the money to go to the Wild Animal Park in Escondido. It'll blow your hair right back!
Quivara Grasslands in Kansas if you're into plains species.
The South Platte River in Kansas in March, for Sandhill and Whooping Cranes and waterfowl.
Reach these places from Kansas City.
Bosque del Apache NWR, New Mexico. Wintering grounds for cranes, geese and ducks. Go in November.
This is south of Albuquerque.
The Everglades are legendary, of course.
And in Florida, also check out the Gulf Coast for Corkscrew Swamp and Sanibel Island (Ding Darling NWR). The Space Coast, on the east, has all the NASA stuff you could ever want, plus Merritt Island NWR, near Canaveral. My brother-in-law STILL gets the blithers talking about how wonderful this place is.
There's wonderful places in every state in the union -- no matter where you go, there will be great stuff to see. I guess it all depends on how MUCH you want to see.