Send him over to spend a month with my family and nothing will phase him.
Seriously, I read all the posts and agreed with this one the most......
I think you are getting a little too carried away with the "things" you want to expose him to, and aren't thinking in a bigger picture. "Things" are a minute detail... just because he sees them at home doesn't mean he will be equipped with the things he really needs to "know."
A horse that is "bomb proof" is one that easily overcomes. I don't know what trainer(s) coined the phrase "ability to overcome" but to me it is an ability that I find important in a horse that I drive, ride, or show.
A horse that can easily "overcome" has been trained to respond to "scary things" in a safe way. The horse learns to trust it's handler so that if it finds an object it knows the handler will help him overcome by either blindly trusting, or having the handler allow curiosity to take hold, or have the handler force him through the obstacle. Those are generally the three ways you can get past an object of fear.
I have found though if YOU ignore something they generally tend to too.
Though we do a work a little bit at exposing our horses to different sights, sounds most of their "bomb-proofing" just happens naturally from being handled a lot and being consistent so that they are more confident and just trust us to keep them safe. We do have a couple of more high strung horses young and new to driving and both are very responsive to being talked to a lot in a soothing sort of baby talk. It sounds silly but one was terrified of a person in a very bright sweater with wild patterns that kept moving around near the rail during a driving class and I definitely talked him through it every time I came around and had to pass the person. He was tense but he was flicking his ear back and obviously listening to me saying things like "ooooh...looky at that funny little person in the crazy colors, isn't he funny..." I'm sure it wasn't what I was actually saying, it was the light, soft soothing voice. I've done the same thing when I see something likely to spook my horse first of all I will act like it is no big deal and go with this plan of action first.......
I have found though if YOU ignore something they generally tend to too.
Horses are usually pretty in tune to how you are feeling and what you are thinking and will check in with you mentally to see if you are tense or scared or fearful and take their cue from you as to how they react to whatever is happening. If pretending it is no big deal and ignoring it doesn't seem like it is going to work, at that point I will start talking about whatever is spooking him and get the horse listening and connected to me so that he doesn't get scared and blow up. "see the silly dog barking? isn't he funny? what a goofy little thing.... "
Having said all that....to be fair, my kids are always around our horses if not working with them, playing around them, riding their bikes in the pasture, scooters in the barn, kicking soccer balls, etc. there is usually a radio on in the barn and people coming and going. I've volunteered to have girl scout troops come to our farm and learn about the minis and handle them so they can earn their horse badges, I take minis to school to visit classrooms (they've even ridden in the school elevator) we had a mini play Rudolph the red-nose reindeer in a holiday program, took them in parades -- which is the ultimate sensory overload---crowds, clowns, sirens, lights, marching band, cheerleaders, etc.
We wanted our hunter jumper horses to be exposed to everthing possible so when we were training them we built all sorts of busy, colorful, scary, wild, unusual jumps that they got used to jumping at home and even ordered some of the jumps used at nationals -- like the brick wall -- and then we found out at a little local show that a plain little wobbly jump with a single white pole was too scary for them to jump over!!!
That shows you shouldn't over think it too much and try to prepare for every imaginable situation or obstacle, just keep things simple, build a lot of trust and confidence and connect with your horse so that when something you didn't think up comes along it is no big deal for either of you.
I was sitting in the cart waiting to go in the ring at the state fair (scary enough with just the crowds) and the fireworks started up BEHIND us. Neither me or the horse was expecting it and we both almost had a heart attack and he literally went bouncing straight up in the air, four feet off the ground and came down with one leg over a shaft, I popped out of the cart talking to him calming him down, he reared up again and when he came down I got his leg back off the shaft and I spun him around to he could see the fireworks and told him what they were and how pretty they are and how they only have fireworks like that when the most special horses are driving -- like him of course -- blah blah blah and two minutes later we went trotting in the ring and he was absolutely perfect for me, almost like he was a little embarrassed about getting scared and wanting to be his very best. He went into the ring at his fanciest trot with his head held high so he must have believed my little story about only the most special horses getting to have fireworks!