The Golden Garden... a collection of intricately shaped metal artwork created by a particularly artistic First One... now the patron of artists. A monestary of artisan priests tends it. Outsiders are welcome to tour for a small fee (nobles get in freely, of course and often get fawning service because the tour guides get to keep a lot of the tips). There's a wooden board with slowly shrivelling severed paws nailed to it... from folks who were caught trying to break off pieces of the artwork (especially the ones made out of gold filigree).
There's also a room with older pieces that have been recovered, a museum of sorts. No one's really sure who made these... and they include a gigantic tarnished metal fork (obviously a human tool).
The Titan's Bridge: a great wooden bridge, wide enough for a mousy army to march down, which extends out into the Lake and then mysteriously stops. It's obviously an abandoned dock, but to the inhabitants of the land it's a gigantic bridge to nowhere.
The Shrikes: a band of nomadic shrews known for raiding the edges of the Empire. Almost as intelligent as mice and wielding crude and/or stolen equipment, they're driven by their hunger to attack, capture and eat mice and rats. Many of them have been wiped out periodically, but the band's size always swells back up again after a few years.