From Leon (awesome place btw) you can daytrip the beach in under an hour.
You could then head down and see Ometepe (it has a beach of sorts...), SJdeSur and then to the Nicoya Peninsula in Costa Rica. You'll see a few more places that way. In my mind a beach is a beach is a beach...so seeing a few at least adds a tiny bit of variety!
You can do the Corn Islands for $180ish return from Managua. Unless you have 3-4 days to kill on buses and boats, flying is the way to go. But with 12 days...you could spend 3 in Leon, 1 in Granada, 1 in Masaya, 4 on Ometepe (all minimums, except Granada which is boring, imho)....heading all the way to Managaua to get to the corn islands for a couple of days seems like a luxury! Daytrip the beach from Leon. It's a nice beach, nearly empty on weekdays, nice bodysurfing waves and easy to get to.

