There is no beach nearby as Chachagua is in the middle of the country in an isolated location. You may be able to get a shuttle bus to pick you up on the main road from Arenal to Jaco or Manuel Antonio but you'd have to contact the shuttle company directly. I'd recommend you hire a private driver. Just do a search on this site for recommendations. You can also take public buses but you'll be looking at a lengthy trip and multiple connections. You should also ask your lodging in Chachagua about transfers as they may have a driver they work with.
The closest beach is at Puntarenas but this is not an attractive beach area IMO. The next closest is Jaco tothe south. I would recommend Manuel Antonio as your best beach option but it's an additional 1-1.5 hours south of Jaco.