Personally, with a very good doer, I stick to hay blocks in a munch net. It you buy them in bulk, they are less expensive and my mare really enjoys them.
A few friends of mine have had good success with Likits and Bizzy Balls. I think like most things with horses, some things work better than others for different horses.
Mine loved his snack ball (filled with celery and rosehips) & hay ball when stabled.
I used to make vegetable and herb hangy-things too from plaited baler twine tied to a high point and poke in celery, swedes, nettles, gorse and other herbs I could find through the holes.
Minto and sultan loved their snack balls. They would get some high fiber nuts each night. Robin however is not allowed. It gets kicked with such power yo wonders what on earth is going on with all the crashing. He then somehow dumps it in his water after digging through his entire bed and leaving bedding stacked up on the wall. I forgot how bad he was, tried again the other day and quickly remembered why he dosnt get it. How he gets it in a big trug tub I have no idea!!
I put some treats in a welly boot, or a treat ball, swede on a string, apple bobbing - all of which i rotate so they don't get bored with the same thing every time. They also have a broom head and a door mat for scratching. I am in the fortunate position that I can leave stable doors open for them wander the yard too, so I also put out small buckets of water with a different essential oil in each (they obviously have plain water too).