Hi my cage is exactly the same size as the Hamster Heaven. Same cage basically but with smaller bar spacing and no tubes or penthouse. I'll add a photo of Charlie's set up below, because basically - if you ignore his roof tube and sputniks, you can set up good floorspace. He has quite a large house with a lift off roof (for checking on him and his potty!) which doubles up as a low shelf. The bendy stick bridge tunnel over the house entraance allows access to the top of the house. It is a huge house and takes up about a third of the width of the cage, but if you don't want to put a potty inside there is a smaller version that is still a good size and makes a good low shelf.
Anyway basically you could stick to deep substrate and floor toys and maybe have a grass hammock hanging down really low near the substrate so he can get in it easily and have a bit of a swing! If you can imagine Charlie's set up without the sputniks (he has a vine branch in the front left corner that could be lower down so he can't climb it and fall). And without the long bendy bridge. The small log under the long bridge is a carrot stuffed log and Charlie spent hours and hours on this for a good few months. First he chewed all the bark off and then spent ages trying t get the carrot out of the middle, so that could be something good to add to the floor of the cage too.
Carrot log roll
http://www.amazon.co.uk/Rosewood-Bor...arrot+log+roll
Grass hammock - you could extend the length it hangs down to with some sisal string maybe.
http://www.zooplus.co.uk/shop/small_...es/mats/204595
I would block off the tubes and take the penthouse off and as Cypher says, use a couple of the tubes as floor toys, maybe partly under the substrate. They tend to like something to sit under as well, so the grass hammock would provide that rather than a shelf.
You could also have quite deep substrate - mine is currently filled almost to the top of the cage base - and hide a few things in it for him and scatter feed half his food and put the other half in his bowl. This should keep him interested exploring the cage and distract him from missing his previous set up! I had one of the coconut topped cardboard hay houses on the floor of the cage (ie under the substrate) to support a piece of granite - then I moved the granite somewhere else and forgot the hay house was under there. Charlie dug the top of the substrate a bit, found the coconut top and had great fun nibbling it!
He has a carrot topped one as well (I think they only do the coconut topped ones at Christmas). So that would be something fun to partly bury and he can chew at it.
http://www.amazon.co.uk/Rosewood-Nat...t+topped+house
This is a more recent set up when he had a smaller house and substrate piled high - so plenty of floor space, plus an egg box which I hid treats in the little indents.
This is his current set up with his huge rabbit house with a lift off top. Because the house is so tall it sits right on the base of the cage so it can't fall on him if he tries to tunnel, but I can still have the substrate around it really deep and it provides a low shelf. Labyrinth houses are also good - and he might like something like that to replace his penthouse house. The trixie leif labyrinth house is the same size as the rabbit house in charlie's cage below (except not as tall).
Rodipet make a great labyrinth house for a syrian - it would fill the width of our cages and also provide a low shelf. It sticks out about 3cm further than my current house though so would take up a lot of floor space in the cage and would be better in a 100cm cage.
http://www.rodipet.de/shop/haeuser/r...rinth-xxl.html
I think you could make a floor level only cage really interesting for him
You can also section of areas of different levels. You can just see on the second photo I have a medium sized bendy stick tunnel as a fence separating a high substrate area from a low one. You could also use a cork tunnel to do something like that as well.