Cork items are good as well - like the cork tunnels - they have a natural texture, can be climbed over or run through. I would put anything cork in the freezer for a week first though to make sure there is nothing microscopic living in it. They can also be a way of separating a deeper substrate area from a lower substrate area. (Check out the photos on the zooplus page)
Tunnels & Dens: bargain prices at zooplus: Trixie Cork Tunnel
I wouldn't put any plants in either - either real or false, because of chew risks and moulds from damp soil. Millet sprays can look like long grasses though and look good. I did try some millet sprays but Charlie ignored them lol!
This pic from pinterest looks nice with millet (see below)
I had one of these sandblasted vine branches from rodipet in Charlie's previous cage as well. It gave a bit of shelter to one corner and could be climbed up and hidden under. The Rodipet ones are a good size for a hamster cage (ie not too big!) I put that in the freezer for a few days to a week as well to make sure nothing was living in it!
https://www.rodipet.de/shop/zubehoer...e-ca-20cm.html
A lot of the resin bridges and ornaments are made for fish tanks or reptiles so a lot of them have fake plants on, but if you like the look of that kind of thing there are a couple of similar ceramic things for hamsters - not sure if some would be big enough for a syrian - I think they would fit but only just!
https://www.amazon.co.uk/Living-Worl...ds=hamster+gym
https://www.amazon.co.uk/Living-Worl...MTEQ5J6BNJ6JB7
https://www.amazon.co.uk/Living-Worl...9VJKYHDG18JZDF
I always tend to think of the safety aspect first, so anything hard like ceramic could do to be under a shelf really, in case the hammy climbs to the top and falls - best if they can only land on something soft, like substrate.