Yes Monteverde is a must and the place I'd highly recommend El Sol if you want wonderful accomodations with friendly knowledgable hosts. We have some virtual tours of their place on the
Monteverde Hotels page. A Zip line tour and hike around Arenal Volcano would be a great one. If you like Hot Springs I'd highly recommend that you also visit
Tabacon Hot Springs this place is most amazing, a bit on the touristy side but all in all an amazing place to visit. Are you planning on visiting the coast too?