I'm partial to the beach, too, and have just finished my little Ushuaia loop recently. Coming south in Nicaragua, definitely turn off the main road and stop into San Juan Del Sur. Then, once you're into Costa Rica it's worth turning off at Liberia and spending a couple of days at Playa Tamarindo, but it IS a tourist trap and the surf's not as good as I'd expected, but still OK nonetheless. From there, it's not too bad a ride to the ferry at Playa Naranjo which goes across to Puntarenas, but the road's a shocker in places. (On my way back North I took the road through Honda, which also had some scruffy bits....roads in Costa Rica are generally pretty crappy actually)
Further on, if you want a taste of the Caribbean, head north then east through San Jose to Puerto Limon and along the coast to Pto. Viejo. From there you can ride through to Panama along the coast, Bocas Del Toro's well worthwhile, but there's two VERY scary railway bridges to deal with. Or, if you want to stick to the Pacific side, Dominical's worth a stopover, and also it's not too hard to ride dirt down to Pavones itself, but watch the weather....we got stuck in a massive downpour and couldn't get through the very last creek which came down in flood.
Whichever way you go, it's all fun but the police in Costa Rica like fining fast motorcycles.