Yes, you really do need the waterproofs. The weather forecast had suggested it would be sunny all week, however it did rain on one day (the whole day) and I was quite glad to have my hat and gloves on that day as well as my waterproof jacket and trousers.
On the free day, go up with the cable car to Aiguille du Midi, then take a return trip to the Italian station and back. You'll get wonderful views of Mont Blanc, but more entertaining was seeing all the people out there in their snow gear trekking along. Then take your lunch at Aiguille du Midi station, the canteen isn't very expensive. Then take the cable car back to the halfway point (Plan de l'Aiguille) between and Chamonix. Walk to le Montenvers, then take the train down to Chamonix. This was one of my favourite days.
The walking was a little harder than I had expected. I think I read somewhere that a general level of fitness is required and if one was used to weekend walking then this is fine. Although we all made it, it's worth being aware that the ascents are quite steep and at times feel a little unrelenting. There were plenty of breaks, however some practice with walking up mountains before hand might be advantageous.