Question: Every time I go walking or hiking, I generally spend most of my time looking down to make sure I don't step on a snake or hole or dog poop. I don't do it consciously, it just happens. Obviously, I miss out on a lot of birds and small animals up in the trees and I'd like to see how other people cope with the urge to avoid obstacles and just enjoy their surroundings.

How can I go walking and hiking without constantly looking down?

Answer: 
I find this is something I do when I can't see much of the scene in front of me at once. This is normally because I'm not far from the person in front of me. Therefore, my advice is: walk in front. Or, leave a big gap between you and the person in front of you. This is perhaps somewhat antisocial, but personally I quite like to walk alone about half the time. It lets me focus on my surroundings a bit more.
I hike with my feet slightly turned upwards against the direction they are moving. This prevents stubbing or anything abrupt and lets me get a firm grip on the firmament whether I see it or not. Also, after some thought, I tend to place my foot from the back-outside to front-inside and let it settle a bit until I feel it catch and then apply weight; for places with loose or jagged terrain, or simply where I want to move with the ground that is really helpful for me.

Personally rather than scan, I sort of observe, which lets me see beneath and above, then too I listen since a lot of snakes I can hear or catch their motion visually. 

To me the ground really isn't overly concerning, if I can see the challenges in general, and don't have to be concerned with the nitty-gritty of minor debris than I am golden.