For a day trip from SD Boca Chica beach has the advantage of having the largest lagoon in the Caribbean, sheltered by a reef about a quarter of a mile off shore. It's a great swimming beach and the water is shallow enough so that you can walk out to the reef.
If you spend the day at the Playa Vista beach cliub just behind the Europa Hotel you can have a pleasant experience at a place run by a great Englishman and a pretty good Dane.
The downside is the many beach vendors but they can be dismissed with a smile and a "no gracias."
There are hookers who ply their trade and that is a turn off if you are not used to third world "ambience" but they won't bother you (while your wife is looking :laugh
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The night scene in Boca Chica can help you understand the concept of "low life".
The other beaches mentioned are nice but Boca Chica town has more options for eating.
$130 round trip for a taxi is a rip off.