All of the horses in our barn are stabled 24/7 with very limited turnout. We can only use the pastures when it hasn't rained and we have to stay on the property while they are out. Also, with 30 horses and only 4 small pastures sometimes there isn't room or time-very frustrating.
When all else fails we turn them out in the arenas to run or one of the muddy round pens but once again we have to stay there while they are out.
For the most part these horses are quite happy as long as they have an owner that comes in regularly to spend time with them. Their stall becomes their security and the source of all good things especially food!
Some do have a tendency to chew or scrape their teeth on the stall wood out of boredom and several have developed stall vices like cribbing, weaving and tail rubbing.
The barn owners don't really know or care much about horses and get angry about their stalls being chewed up but they just don't seem to put two and two together.
Most horses get bored with stall toys and some don't play at all but the one thing I have seen that usually works is the Jolly Ball that dispenses treats. The horse has to really work at it to get them so it's not like they are gulping down a lot of them. The other one I have seen that horses love is the one that the horse rolls on the floor that also dispenses a few treats a little at a time: Nose-it! Ball Funnel Slow Feeder & Treat Dispenser for Horses - ShanesTack
Don't worry about noise-in reality it doesn't bother the other horses at all.
There is also a cheap thing you can do for a bored horse that constantly wants to mess around with things. Every other day I take out an empty plastic gallon milk jug (remove the cap and any other pieces) and just leave it in the stall. This only works with one of my horses but he loves to stomp and crush them. They only last a day or so and are smashed flat.
They don't lose any pieces so they are pretty safe.
Slow feed hay bags help too-the horse can work at it all day long and really aren't getting as much to eat as they think they are!
Being kept in a stall may not be a terribly natural thing but it doesn't have to be a disaster. Just try to keep them entertained and remember that this becomes their "safe place"-and that is very important to a prey animal.