A lot of toys etc sold for hamsters are a bit useless or too small or sometimes a bit dangerous - just to warn you not to spend loads on such things - I learned the hard way with some things. Zooplus have a reasonable selection. Rat sized tunnels and sputniks etc are better for Syrians. Amazon sells some things and so does viovet. To be honest things like cardboard are good - an old tissue box as an extra hide or a kitchen roll inner tube slit down one side. And a shoebox can make a good house/nesting box (you can even put the litter tray inside at one end - the door end is best as they tend to nest away from the door where it's darker). Cut the base out and a hole for the door and keep the lid as a lift off roof for checking inside (and for emptying the litter tray). But other houses are ok too and then the litter tray just goes in the cage where the hammy chooses to pee.
I have a few favourite toys etc for Syrians. I like the large cork logs - they fill a lot of floorspace and are light so don't sink in the substrate, plus make an interesting texture to climb over. Zooplus sells one but it's pretty huge I think. I got the ones from Rodipet because they're shorter lengths - but they work out quite expensive. Anyway here are some of my favourites
Hyacinth tunnel - the small size is a good size for Syrians - not too big, not too small - also chewable.
https://www.amazon.co.uk/ROSEWOOD-Bo...yacinth+tunnel
Rat sputnik - works well hung next to a shelf so they can get in and out easily - and put a bit of bedding in it. You can also tie a rat tube to the roof of the cage leading out of the other side of the sputnik and leading onto another shelf or into another sputnik and make a kind of roof run
https://www.zooplus.co.uk/shop/small...ls/dens/209851
Savic rat tube - this is the one I tie to the roof between sputniks. But don't use the chains as hammies feet can get caught. You can use sisal string which is safe to chew - to tie things to the bars or sometimes I use plastic cable ties. Not necessary really and a bit of an extravagance. Amazon also sell this
https://www.zooplus.co.uk/shop/small...at_toys/132219
Trio of balls. These are good floor toys. I only use two as think the wicker one has sharp bits. The hyacinth one is good - chewable - and you can stick pumpkin seeds etc under the folds occasionally so it keeps them busy getting the seeds out. The sisal one I unravel and use as sisal string for tying things to the cage!
https://www.amazon.co.uk/ROSEWOOD-Bo...=trio+of+balls
Bendy stick bridges - always useful. This is my favourite one because it can either be screwed to the bars or the screws left off and used as a normal bendy bridge. I also prefer the smooth wood - some bendy bridges have gaps where feet can get caught and sharp bits of wood - I got fed up of buying them and getting bad ones. To make a longer ramp attached to the bars you might need two. It can also make a little shelf.
https://www.ebay.co.uk/itm/BRIDGE-HO...sAAOSw5VFWHRqJ
It can get expensive buying toys. If you could only get four things I'd get the hyacinth tunnel, trio of balls, a sputnik and the coloured bendy bridge