The Canadian Centre for Policy Alternatives has a pretty good report here (pdf). Runesmith has a good post on it.
An EI boost seems like a good idea, people aren't likely to just save that money if they're unemployed. Further, if more people keep their jobs than expected (say, due to other stimulus) there will be fewer EI applicants, right?
Similarly, help for poverty action, low income seniors and the working poor would be spent not saved. And those people would need help the most.
I really like the Pension Guarantee Fund idea. I expect anyone close to retirement might be pretty worried about their pension now, making an excuse to cut their spending. Further, if a pension fund fails it sends a BIG message about lack of confidence in the economy. It's the same situation for retirees with RRSPs, but I don't see how you could help boost RRSP value except through general recovery of equity values. Anyway, if someone does retire it potentially opens a job for a replacement.
Affordable, green housing and renovations for seniors would directly stimulate the construction industry to offset any decline due to residential mortgage difficulties. And I think there is an increase expected in the proportion of the population as seniors, so this would be a good time to invest ahead of that trend.
Even if infrastructure doesn't provide the stimulus we all hope for, it still makes sense to buy it while it's cheap. As well, there have likely been many technological advances (green and otherwise) since the last infrastructure build out that should be of advantage now, whether in cost, quantity or quality.
Tax cuts would only help those who're paying tax, which are the most likely to save any extra for a rainier day. Specifically I think past GST cuts weren't much of a stimulus; more of the same won't be any different.