As "Winter Texans", and proud of it, we love the area. There are many fine parks in that area.
There are a bunch of us MOC'ers that gather the 3rd Thursday of every month and have lunch, jabber, talk, share, laugh and have fun!! we post that info under the RGV area on this forum, camping clubs.
I am unaware of any elderhostel programs, but, that might be interesting, if you find something, please post it.
We are treated well, we love the Mexican food and culture, there are programs at many of the campgrounds, musical and such, usually small fees. Most campgrounds have so many activities planned you could not do them all. Some have LOADS more, some have less.
Go to one of the camp ground books, and study the ratings and accommodations. Pick one that suits YOUR needs.
We prefer, no demand, concrete or blacktop pads, no grass. If it rains even a little down there, it turns immediately to mud. (and this is MY opinion, not a slam on other MOCers that choose otherwise, each to his own, ok??
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Be prepared for WIND. It is part of the total package, you will love it, hate it, or put up with it. It keeps away the bugs and keeps ya comfy. It also rattles awnings, and blows dust in open windows. Again, comes with the territory, love it, hate it.
There are some great museums, up and down the RGV, a world class zoo in Brownsville, South Padre Island, birding, birding and more birding.
If you wanna be busy you can be, if you wanna be lazy, you can do that too!
Have a great time, and hope to see you at one of the Thursday luncheons.