There's another great collection of life science activities found at Access Exellence. I have spent most of my time going through the Fellows Collection (1994, 1995, 1996), but the rest of the site is worth looking at as well. As with all my favorites, some individual activities will be featured in their own posts.
The activities found in this section of the website have been submitted by high school teachers participating in the Access Excellence program. As such, the activities were designed for high school students. However, many of these activities are very easily adaptable (or usable as is) for lower grade students.