Did the hike from the La Fortuna side, headed up to the lagoon, and came back down. As most people commented, it was very foggy the entire time; we couldn't see the Arenal volcano, and could only see the other side of the Green Lagoon briefly when the wind blew some of the fog away. As a result, the view wasn't particularly impressive anywhere in the hike, but it was nice to get some exercise and to be able to jump in the lagoon at the top.
I hike a lot in the rockies, so thought it was quite an easy hike; one member of our group was sick, and was still able to make it up no problem (went very easy and was up in about 2 hours). You definitely don't need a guide to do this one. The trail is well-marked; it'd be hard to go wrong. We did see one snake though (tiny guy in the bushes on the side of the trail), so good idea to wear closed toed shoes, but as in general in Costa Rica, watch where you step, and you will be fine.
There was a $10 entrance fee, though that didn't appear to be enforced.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.