My kids and I just went to the SEE Science center in Manchester, NH and they had a really fun exhibit on Simple Machines. The hands-on exhibit was really well thought out and interactive. Here is the lever where you had to move the white lever to balance the heavy object on the right. Not as easy as it sounded - it took us more tries that we expected to balance it properly. Then we played with the pulleys. They had two pulleys that you sit in and pull yourself up. One pulley was a 1/5 ratio and the other a 1/7. This means that they reduced the amount of pull required by different amounts.