El Hacha Falls is one of the most popular excursions in Canaima and is really worth it. It's only a few minutes from the town and you can also go by walking around the lake, perhaps a more interesting option given that you can see the falls from different angles and see the rainbows.
What you can't miss, however, is the chance to see the falls from inside. During the rainy season, it's practically impossible due to the incredible quantity of water. I'd suggest checking out the video I uploaded to see the noise and the power of the waterfall (and that's taking into account that we went during the dry season).