In reply to Chris Ellyatt:
I strongly recommend the trip up the Midi. On the way back down you can get out at Plan de L'Aiguille then walk for a delighful couple of hours along the Grand Balcon Nord to Montenvers; your cable car tickets are valid to go down on the train. Approaching Montenvers the route splits and you have a choce of easy and scenic 500 ft climb then descent or no climbing but more exposed straight around the rocks. We did the latter with a child but he did choose to slip on an exposed bit.
A day in Megeve would be nice, it's a pretty village with a great pool and dry luge. Also if you've got a car a trip round to Samoens (market day best) then up the valley to Fer a Cheval. And as you've mentioned, Annecy is great for a day out.
One superb walk would be from Sixt Fer a Cheval via Anterne and the Brevent cable car back to Chamonix, which would require a night in the Chalet D'Anterne, a pleasant introduction to Alpine walking.
Edit: Balcon Sud faces south and Nord faces north.
Post edited at 13:54