If you want to step up the quality from the 18-105, get a Tamron 17-50 f/2.8..... new they're £300 for the non-VC version. Great lens, optically brilliant.
I've just moved to a Nikon 17-55 and it's brilliant but they're expensive - £600 used, £900 new. Better build and faster AF than the Tamron but little between them optically
For landscape work, personally I'd go for a Nikon 12-24mm f/4 if you're flush (£450 used, £750 new) or a Tokina 12-24 mm f/4 (£250 used, £400 new). Personally, I'd get the Tokina if you're in a budget - difference between them us minimal. You then have a proper wide lens for vista and both take filters on the front.
If you want a telephoto then get a Nikon 80-200mm f/2.8 AF-D.... Amazing lens and you get them used for about £450-500 for the later two-ring version.