We plan to do a wildlife refuge center, canopy tour, and lava hike in Arenal. In Jaco we plan to do the Jungle Safari tour and an ATV tour.

Just me, but for an 8 night trip I would only do 2 stops plus your last night at Xandari. You will probably be getting to Arenal late, and will only have 1 full day, leaving early the next day to get to the Pacific side.

I would probably do Arenal 3 nights, MA 4 nights and relax a little. Plenty to do in both. Villas Caletas looks beautiful, but those TA reviews would make me leary. Definitly do a small tour of MA park, and hang on the beach inside the park. Maybe a day trip to Dominical.

I agree with tqluvtotrvl: cut out Jaco for all sorts of reasons including too much movement and give one additional night to each of La Fortuna and MA. In MA, do a morning with a guide in the park (closed Mondays)and spend the afternoon at one of the beaches inside the park; take a morning cruise on a catamaran do an afternoon on a mangrove tour; spend a day to drive to Dominical and do some surfing and drive back.

for 8 days split time at Arenal and MA, skip Jaco

in MA tour the Nat Park with a guide, enjoy the beach, have cocktails on the hill and watch the sunset

in Arenal, zipline, rafting, night lava hikes...

lots of activities in both places

good choice for Gaia in MA

give Arenal and MA each 4 nights, skip Jaco