Navwar makes a small selection of 1/3000 harbor pieces.
Skytrex makes the Davco 1/3000 scale Harbor Accessories.
Irregular Miniatures makes a large range of 2mm terrain, much of which I find to be undersized and a reasonable fit for 1/2400. I bought most of scenery features appropriate for Age of Sail and decided that nearly everything with a "BG" code looks okay on the shore of a 1/2400 naval battle, but many of the features with "IKS" codes look too big (more appropriate to the stated 2mm scale, or 1/1200). I never bought any of the more modern stuff (railroads, trenches, tank traps, pontoon bridges, etc.) so I can't comment on it.
WTJ makes a variety of shore batteries in 3D-printed plastic and the old pewter 1/3000 range.
Microform Models offers buildings in various scales, including 1/2400 scale. Since this company's primary market is scale model makers (e.g. architectural models) I expect them to be true to scale, but I haven't ordered any, so I can't comment on how well they mix with any of the above items or 1/2400 scale ships, many of which are not very true to scale at all.
For bucolic countryside (woods, forests, farms, etc.) I would suggest scratch-building. You can make convincing forests and farm fields by just gluing varying coarsenesses of ground foam and sand onto shapes of cloth. Use thin cloth (not felt), and it will be able to conform to hills and valleys.
I recommend using very thin material for at least some of your coastline. In 1/2400 scale, anything steep rise more than a couple mm thick is a cliff. I made a bunch of coastline out of 1/8" thick vinyl floor tile, and the sections I sanded into gradual slopes made decent beaches, but the sections I left steep I drybrushed gray to make very convincing steep, stony escarpments next to my 1/2400 ships.
- Ix