Punta del Diablo is on the northeastern coast of Uruguay. If you search the forums for it there were some detailed descriptions posted recently. It's quiet, but fun, and there are some bars and boliches happening that shake things up a bit. Depends on how much you need external stimulation to enjoy yourself. Punta del Diablo is also close to Cabo Polonio, which is a lovely little place that is home to sea lion colonies. I'm sure there's lots of information on the internet about all the aforementioned. Know that going now is low season still, and many of the beach towns in uruguay only get happening with action in January/end of Dec and will be fairly quiet up until then.
Pinamar and Villa Gessel are also nice escapes, but both are much more towns on the beach (Gessel has more going on in terms of a boardwalk, shopping, arcade, etc.) than Punta which is tucked more away (no ATM in town). I'd prefer any of the aforementioned and Mar de los Campos to PdE.