Hey! I love this document and would use it for any of the characters listed, but I noticed there are still no giants and was wondering if you are still planning on doing that.
I could get into the science, but let's keep it simple. A book hits you falling at 5 miles per hour. Very painful, could POSSIBLY kill you depending on a few factors. Now a car hits you falling 5 miles per hour. You are dead, period. And both these objects could fall much faster and we all know...
I feel like the grid is the better of the two options, and I feel like 'static' objects like you have mentioned here, or at least relatively static, have a simple solution. Either roll Initiative for the 'static' environment, make a standard, like always on Initiative 20 or Initiative 1. Then...