youre kidding right? It's definitely high on my list, from what I've read the place is infested with tarpon around Barra Del Colorado / Rio Colorado, Parismina and tortuguero as well as any inland lagoons.
From what I hear it's also one of the best destinations to chase roosterfish with a longer season than other hotspots and bluewater action is hot (but it sounds like you're not going to book a boat).
Looking at maps of Manuel Antonio I would say two things - anywhere there is a fishing boat dock where they clean fish or a restaurant on the water there will be tarpon that will make your eyes pop out. Rio Savengre to the southeast has a large estuary lagoon I am pretty sure has tarpon. There is a road leading to the shore to the northwest of the river mouth that you might work over from but I'd be careful always in foreign places you haven't been to before. There might be saltwater crocs or some crazy stuff who knows
I'd pack a travel 9wt or 10wt. and hit the easy money around fish cleaning docks or better yet find lagoons full of mid-size tarpon at first light before your family obligations since it seems you can't explore the region with flyfishing as the priority.
Super heavy leader, razor sharp hooks and be glad when you jump them even if they break something. They are awesome fish.
Who know maybe a pod of roosters will strafe your beach...