How to work with an overly positive advisor?

I sympathize with this because I had an overly positive advisor for approximately 3.5 years, until I was getting ready to go on the job market and then suddenly he had tons of problems with my work. Others have told you to go elsewhere for criticism. This is probably a good idea, but you should also be able to have a good, balanced discussion with your main advisor who knows your work best.

Two things might be helpful:

1) Ask her what criticisms a reviewer might have of your approach to X

2) Ask her what needs to be done to improve Y

The first may help because she may be able to get in a different mindset about criticism if she is asked to think about the paper like a reviewer (a reviewer's "job" is to be critical, but she may feel that an advisor's "job" is to be encouraging). The second may help because it invites her to frame problems in an encouraging, positive way (which seems to be her comfort zone).

It seems to me that the problem isn't her being positive, it is that you:

get the same exact level of praise on the weeks I do a lot of work and weeks I do little work.

If she calls your work 'great' on good weeks and 'pretty good' on bad weeks, you can work with that. However if she says 'good job' in both instances, there is no way differentiate between good weeks and bad weeks.
One way to deal with that is to explicitly ask her to compare your work with previous work. "How does this paper compare with my previous papers in your eyes"? "How does my work this week compare to my work in previous weeks"? "Would you say these results are a more significant contribution to the field than my earlier results X, Y and Z"? Emphasize that you want to know when you did better then other times to help you learn and what to emphasize in publications/posters. Hopefully she will respond along the lines of: X, Y and Z are all great results, but Z really stands out. Still very positive, but differentiating between the good and the great.
Another option would be when considering major results/things you want to publish is to ask other senior academics for their opinion. In this case, remember to keep your supervisor in the loop. A way to approach it could be: "I am really proud of this part of my work and you agree that it is great. Do you mind if I show it to person X and discuss it with her/him"?