Thank you for your feedback, that really helped me.
On the Telezoom
I will get the Sigma 28-200mm f/3.5-5.6 next month -- should wait until my birthday but was tempted to get it early for the lakes. I found out how difficult it can be to decide on a lens. Like you say it would be a great focal length for shooting there.
Yes I will take along the kit lens, this is a no brainer for me. I've really got used to it and the 18mm will be very useful for landscapes. It seems like you say the 50mm is very much a specialist lens although I am going to see how far I can push it.
As for low light, I suppose it's still a compromise because the lens only performs better in low light if the aperture is wider open and that means a shallower DOF. Still 3.5 at 50mm should provide a good amount of detail.
I have a Canon bag for the camera but I don't think it's waterproof -- it's the standard one you can get in a kit -- although I bought it separately. I would put it away if it started raining but my worry is if we went out for a day's walking/hiking the weather can be very unpredictable. Any waterproof bags you can recommend for cameras?