Hanging ladders - any runged ladders
Anything with chains (the chains can be replaced with sisal string)
Anything with a window or opening smaller than 5cm (6cm or more ideally)
Anything fabric - hammocks etc. Fleece is ok but hamsters can still chew it or get their feet caught in eyelets. I tend to only use fleece toys out of the cage - very occasionally in the cage with an older hamster who doesn’t chew
Most things sold for “hamsters “ - they are nearly all too small for Syrians and only suitable for dwarf hamsters
Kapok (same effect as fluffy bedding)
Tbh honest in the end I looked at what hamsters could safely have and ignored most of the things sold for hansters.
So what they can have is
Cardboard - tubes, tissue boxes, egg boxes, shoe boxes - all make good hideouts.
Chew sticks
Rat sputniks
Grass hammocks (but check for sharp bits)
Bendy stick bridges
Rainbow bridge (they all seem to like that)
Cork logs and tunnels
Millet sprays
Any ceramic items (providing entrance big enough)
Wood houses (although check for nails and glue - some are very badly made)
A shelf
I tend to stick to those basics
You can make a nice hammock out of toilet rolls and sisal string as well