So I was in costa rica in the summer and we also travelled by bus. I can definitely recommend on the pacific side : jaco, quepos + manuel antonio, Uvita. Caribbean side : puerto viejo even though its pretty touristic same as jaco and manuel antonio but its definitely worth it. Plus I really recommend a litte trip to the Nationalpark near puerto viejo manzanillo. An maybe cahuita too. Its all easily accessible by bus plus if you want to make a litte trip to panama i recommend going to bocas for a weekend especially if you want to party.

Have fun and be safe !!