Hi Grant - I can see how this must be incredibly frustrating for you. When anyone we love is in pain we'd give anything to 'flick a switch' and get them out of it.
I do also see if from your wife's point of view though. As I am sure you are well aware, when we are depressed we feel COMPLETELY isolated. I am also very fortunate in the fact that I do have a wonderful fiancée who has stuck by me through all of the Peter stuff and all of my depression - and he is very practically minded. He has, thankfully, never had depression, but I know he finds it incredibly frustrating when he can see that, for example, if I was to go out with him and the dog in the evening, it might help lift me a little and it could actually help me sleep.... BUT my mind is saying no. I know, practically, he is right. But because I have no motivation, because my mind is going 'yeah - might help for like... an hour, but after that, you are gonna be feeling exactly the same...' I say no, he gets frustrated, he goes out, slamming the door behind him and I end up feeling worse because now I've upset him when I know all he is trying to to is help!
I think all you can do is be there for her. Suggest things that you can do together - but if she says no then you need to realise WHY she is saying no. You have though, put that idea into her mind, she may not do what you've suggested today - but on a 'good day' she might be slightly more receptive to it because its not been sprung on her out of the blue. Depression is such a frustrating illness for everyone involved - not just the person that is suffering. People on the outside can SEE what would maybe help, whether they have been there or not, but you have the fortunate insight to know how isolating it does feel to be fighting against the constant thoughts and questions in your own mind - try and imagine how it would be if the situation was in reverse. If you were at your lowest low and your wife, with all good intentions was trying to lift you out of it because she had been there before.... I know myself that in a way, it would put added pressure on me to feel better because what has been suggested has worked for someone else, if it then doesn't work straight away, is she then going to feel worse?
As I'm sure you are well aware, recovery is all about 'baby steps' - taking one thing at a time and overcoming it. She will get there with your support - but, as frustrating as that it is, thats all you can do - support her. To someone who is so overcome with depression that they cannot see a way out, support really is the best thing. Just someone to curl up with and watch a film and try and escape for a few hours is worth the world!
Good luck! :)