1: You may have already thought about this, but how about making some figures in the shooting range that shatter when you hit them?
2:Try adding some scuff marks or mould or water to the blank concrete (like in the mine cart track screenshot.)
And some questions:
1 Will there be a underwater tunnel that goes under the island?
2: How long will the cart track be?
1 - Already done, the figures I used can be shot and they will shatter.
2 - I could use a different sort of ground that doesn't look like concrete (without raising the object count)
3 - The underwater tunnel I've made enters under water in the north and leads to the tunnel system. There will be another tunnel leading out of the island under water (when I can make it work)
4 - The cart track is about the same length as the main prison complex (could be a bit longer)