I paid $4 on top of the $10 entrance fee to have access to the Canopy Walk and the Emergent Tower. I think these attractions are a bit overhyped. It was certainly cool to be 120 feet in the air above the tree line, and the view was all the way to the lake, but the experience did not add much value. The Canopy Walk was even more overrated in my opinion. Certainly worth $4, but you won't miss anything by not doing this and this isn't something to do every time you go.
My highlight was the Holden Butterfly Garden. This grass pathed garden was simply gorgeous. It had two bridges, benches, two ponds and was simply beautiful. To get to this area walk through the Visitor Center.
My two biggest cons were lack of shade and poor signage. There were signs everywhere, but they didn't directly relate to the map I was given and it wasn't always clear what trail I was on as compared to the map. Individual attractions were well labeled, but the general trail was not mentioned often. I would suggest the HA invests in a GPS based map.
Overall this is a nice park. It is very clean and well maintained and there are many separate ares to entertain time and time again.