Principles
Helping kids learn to observe and quantify
Measurement
Measuring is always fun, but better to find ways to measure unusual stuff. There are easy kits to make clinometers to measure the height of stuff (like trees, buildings, basketball hoops, model rocket altitudes, etc). Estimating would be good too: using samples or mark-and-recapture strategies to estimate frequencies in populations (of, say, beads or marbles in a jar). Or do it with people: go out and mark people's name-tags, then go out and count how many name tags have marks and use it to estimate the number of people in the gym.
Physics
Make a really large pendulum in the gym. Demonstrate conservation of energy (you can do an effective demonstration by having a kid stand on a platform and let the pendulum swing from their nose out and back -- it will come back and almost touch their nose, but looks terrifying when it's coming toward you.) Use to show coriolis force and precession (due to rotation of earth). Have kids make predictions about effects of changing parameters of a pendulum (e.g. length and weight) on it's period.
Chemistry
There are a bunch of quantifying things we could do: predict how much CO2 is evolved by different quantities of vinegar and baking soda: discover linear relationships and limiting factors. Discover relationships between solubility and temperature.
Biology
If we're willing to have kids do dissections, we can buy frozen squid very cheaply (and then make calamari, if we want!) Squid are wonderfully unusual to dissect. I might be able to get some extra rats or other animals left over from the intro labs this year: we probably wouldn't have enough for kids to do their own dissection, but we could have some opened up for kids to see various structures and to compare with the squids.
We could set up some forensic case studies using microscopes: the story says a bank robber is caught but has hidden the loot somewhere -- some pollen is found on their jacket: is it from the mountain, the field, or the seashore. You can easily do that same kind of thing with hairs or other fibers. It will take some work to get the case studies set up.
It might be fun to get a couple of mammal skeletons and then give kids bones from random other mammals and have them try to figure out the homologous bone and, by estimating the size of the animal, narrow down what kind of animal it might be.
We might be able to get some reptiles -- maybe a big snake (minimally we have a little snake that kids could look at and handle).. Someone should contact the entomology department at UMass -- they have (or had) an insect outreach program that might be able to bring interesting insects.