I think the absolute basics that a dwarf hamster needs in their cage would be at least one large house, a sand bath with enough space to dig around and a wheel, plus of course food and water and enough general enrichment in the forms of hides, tunnels etc to fill up the rest of the space.
It's good to provide some different levels with platforms, though nothing too tall for a dwarf hamster.
I would say at least a couple of small hides such as coconuts or tealight holders (or similar) are good as they often to eat or just sit in these, more if you have space.
Tunnels are important too, such as cork logs, cardboard tubes etc. You can bury these to help them with burrowing or place them at the surface to provide overhead cover.
It's also nice to provide some different textures for variety. So aside from the main substrate, sand, and probably wooden platforms, you might include a terracotta tile or pot, other types of ceramics, hemp or grass mats, or moss (from a pet shop, not collected from outside). You can also get dried leaves for rodents and many people use dry coconut coir though this can hold onto moisture so is best used in a large and well-ventilated enclosure. Cork logs are very useful because they also provide an interesting texture for the hamster to climb on.