Pinamar / Carilo you can do a tour that is 4x4 driving across the dunes, followed by sandboarding (snowboarding on the dunes), followed by some other activities. You can also rent quad bikes and go up the dunes.
We did this off-season one time. Other than that it was pretty dead. There is a place for fantastic hot chocolate and that's about that.
Even on the long weekend (the 9th of Oct) there will be a few more activities but not a tonne, simply because it's still really cold out on the coast, so not a lot of people around. The "bohemian" markets are open and you can have a little browse around.
Make sure you bring fleece, hat, glove, scarf etc if you plan on walking on the beach -- the wind is freezing this time of year.
But don't despair -- we usually end up out at the coast in the off season too, just because my mother in law has a place out there. There's not a lot to do but it's relaxing all the same, play some tennis, go for a walk, make an asado etc.