Tunnels (as mentioned below), sewage systems, foundations of large structures, detritus, glass and ceramics. Most modern concretes would decay quite rapidly on these scales, metals would disperse, plastics get eaten.

The patterns we have made on multi-kilometer scales will remain for very long timescales. We have altered floodplains, water tables, and ecosystems so much so quickly that we will recognise artificial desertification for a very long time indeed.

And we'll still be confused about whatever the fuck we were up to, other than enjoying the human condition :)

Reinforced concrete is probably more durable than unfired clay.

Unreinforced concrete will last a lot longer than reinforced concrete. Steel rebar rusts away.

Regarding clay vs modern concrete formulations, my money's on the clay.

Steel reinforcement is only one type. You now have fiberglass reinforced rods or you can use an additive like fiber mesh.

If the steel rusts away, you just have unreinforced concrete.

You have unreinforced concrete riddled with holes, in a shape designed to depend on the steel. It won't last nearly as long as a structure designed with unreinforced concrete in mind.

Anything made of stainless steel and plastics.

You mean all of those religious artifacts?

How else would you explain Anime figurines in 3400 years? :)

Tunnels perhaps.