Depending on what kind of equipment you have access to, bio can be extremely hard to be able to "see". Certain macro-level things you can, but most of high school-level bio is harder. Even for things you CAN see, looking at onion slides through a microscope to visualize the cell cycle isn't very great. A lot of the visualizing in bio classes come from models or simulations (lets play a game that mimics natural selection!)
Physics teachers I know are always doing demos and really cool labs to help visualize the concepts (one guy in my hall has a bowling ball hanging from his ceiling, last week they were coloring ice cubes for convection).
I'm a bio teacher, so I might be a liiiitttle biased here. I'm also teaching at the basic high school level (standardized testing) at a school that does not have the best funding in general with little access to lab equipment (sequencing DNA would be awesome, but we don't have PCR machines)