Generalizing is so incredibly important. As all of you have said, a behavior, which is solid on the spot where the dog has learned it, does not mean he knows it on another spot. So, generalizing in different surroundings is one part. Generalizing with different objects is another part.
In my training lab I use in between lots of different items, different beds, chairs, benches, big and small, round and square perches, a household ladder in the way we use it ourselves climbing up to the platform, but also a ladder just laying on the floor, so the dog has walk over it, once putting his paws between the different steps and once putting them directly onto the steps. I also use boxes and boards of various forms, lengths, heigths, barrels, balls, also one of those big exercise balls humans use (great exercise for their sense of balance), halfball, wobble ball, skateboard.
There are so many things around us we can use, without spending a lot of money. I also use myself as an item. Eg. I put myself on all 4s and the the dog steps onto my back with his front feet (not necessarily the whole dog, question of his weight!), Or I kneel down, but one leg elevated (placed on the foot) and the dog steps on my thigh, also on my hip or my stretched out arm. There are innumerable possibilities. (For using our body I'd recommend to do this only with a dog who knows something like a "get down" command first! Some dogs have even kind of inhibition to mount the owner's body and for them we will need more patience to overcome this.
A session which all my four dogs love immensely is when I create a little parcours with different items they already have practice with, but all in a confused order, changing it every time randomly, so they have to know very well the command you use.
Another great part, which already has been mentioned here, is generalizing in the outside world, on walks for example. There we will find such a lot of possiblities, sidewalks around houses, big stones, little hills, elevated parts of a roadside, benches, tree trunks and so on.
It is all fun for the dogs and perfect for their body awareness. About body awareness: with a bit more advanced dogs try also to teach him step onto some items with his hind legs first. Best way: Teach him first simply going backward on flat ground, you walking in front of him, baited fists in front of dogs snout, TM and reward in the beginning for each step he takes backward, later on use duration marker and reward only randomly.