We would suggest an all-season tyre for your Nissan, which would give you good grip in all of these conditions, and with the relatively low mileage they won't suffer by being driven on normal roads.

You don't mention your tyre size, but assuming they are 215/60R17 they are plenty of options. A quick look on www.blackcircles.com brought up options such as the Yokohama BluEarth-4S AW21 or the Avon AS7 All Season. You could switch to a proper all-terrain tyre such as the Cooper Discoverer ATT All Terrain which would give you the best grip in tough conditions, but it would compromise the performance on-road so for only occasional use off-road you may find them too extreme.

It is not wise to mix different types of tyre so we would suggest replacing all four tyres at once, but one option would be to get a second set of steel or alloy wheels and fitting your new tyres to them. This way you can switch between tyres as the seasons require and get full use of both sets, rather than scrapping your existing tyres.