I'd concur on the notion of a Sigma 10-20mm f/4-5.6 - can be quite a useful lens for some landscapes, and won't break the bank. If you go that route, you might also want to obtain DxO ViewPoint, which DxO is currently giving away
over here, until the end of June - that'd allow perspective correction quite easily. For landscape work, consider a circular polariser, to bring out the best of skies and water, as well as reducing reflections from shinier objects like leaves. Bear in mind lenses come in various diameters, so you'll either want to buy one for whichever lens(es) you want, or go with a "universal" solution like Cokin's, where all the filters are larger than required, and use adapters to hold them securely in place. I'm primarily wildlife, so I just have a single filter suitable for the 10-20mm; but, you might also find utility in a graduated filter, to reduce the impact of a bright sky on a scene, and possibly even a neutral density filter, for prolonging the exposure time on a shot, for anything from "softening" the look of moving water, to even removing people and other moving objects, with a long enough exposure.
On the mapping front, I've been (mostly) very pleased with Ordnance Survey's MapFinder app. The cost of the 1:25000 tiles works out comparably to the paper maps, but rather more convenient, and they won't fall out of date. The tiles are (or can be, as per your choice) downloaded to your device, so there's no need for a signal.
Another possibility that comes to mind is a set of extension tubes, if you think you might want to try out macro work while out and about - not an overly expensive proposition, at around £40 for a set that'll carry the signals between the lens and body, so you're not stuck operating fully manually.
For some scenes, you might also want to stitch photos into a panorama. On the free side of things,
Hugin generally works quite well; paid, I've been happy with
AutoPano; both are cross-platform, available for OS X, Linux, and Windows.
Most importantly, however - go out often, and use the camera plenty. ^_^ You'll soon find out where your equipment priorities lie. In my case, it was moving from my first lens, a Tamron 28-300mm f/3.5-6.3, to a Nikkor 300mm f/4D, as I'd realised I was spending most of my time at the far end.