Getting to Nendaz:
Most prefer Geneva and Turin airports for their wealth of reasonable flights and ease of onward travel to Nendaz by car (~150km / 2.5 hours drive), but Zurich, Milan and Basel are all doable ( 4 hours drive) and there are infrequent expensive flights to Sion airport itself.
A car is not necessary within the resort, so an alternative approach is to take a train (like clockwork) from Geneva airport to Sion and a bus (45 min) or taxi (30 min) up the winding road to the resort.
Skiing:
Nendaz is in the middle of the extensive 4 Valleys/Verbier ski area. Ascending to 3300m, the area has an excellent snow record and the 412km of pistes offers skiing for all levels.
Nendaz has the “Families Welcome” accreditation. The local slopes (Tracouet) offer comfortable ‘safe’ skiing in a very attractive environment with a fine restaurant. There is a local ski pass available and a range of reductions for children and families. For intermediates there are a few red runs which, being North facing, keep the snow well. For experts there is wonderful uncrowded black and off-piste skiing under the Plan du Fou.
Importantly, there is excellent access to Siviez/Tortin by ski, bus, or car (fastest at ~20mins), which is probably at the heart of the best skiing in the area. One can take lifts to Mont Fort or Verbier or head East towards Veysonez and Les Collins, affording a fantastic choice of terrain, aspect and snow conditions.
With a car its also practical to ski days in nearby resorts.
In the region and Summer action:
Summer in the Alps offers fantastic walking. There are many routes demanding varying ambition and fitness, including numerous more gentle walks along the bisse created to distribute water to the orchards and vinyards. One can even take a lift up and walk down! There is a considerable network of routes suitable for mountain biking- one can hire bikes at Neige Adventure.
Nendaz has facilities for many other summer sports including tennis, volley ball and outdoor swimming, and runs mini-courses for children in climbing, archery and horse riding (at least in 2009) as well as organising walks and visits to local attractions and festivals such as that to the Alpine Horn Festival in July.
Sion is well worth a day out for the walk to the castle, the cafes spilling onto the the traffic-restricted streets and a cycle ride along the River Rhone (rafting available too). Most summers the galleries and museums have special exhibitions and recitals- details from Sion Tourist Information.