Thanks, Amy!
1. While a SMALL kid (to me they are ALL small...) stands in front of you with skates on both you and the kid, push on the kid's shoulders with enough force to cause motion. BE CAREFUL, small children sometimes don't need lots of force to fly across the room! Note, the kid moves backward a much larger distance than you. Keep in mind, I am 250-ish. Most of my kids are in the mid 100's. This works for me all the time. If you are on the small-ish side, pick a LARGE kid and YOU move, not him. Same effect, different direction.
2. Do this in reverse IF the kid and you (or two kids, if you choose.) are OK skaters. Head toward each other with enough speed to actually notice a change. It doesn't have to be blazing speed, but fast enough to note a change in speed when you collide. Grab onto each other upon contact. Note, if the mass difference is large enough and the speeds were the same, the "group" now moves in the direction of the larger person. The little guy got "gobbled up".
3. HOVERCRAFT - I use a home-made hovercraft, see my hovercraft page, to demo 1st Law. This is absolutely GREAT due to the "lack" of friction and it's actually quite dramatic!
2nd Law Specific Demos
Cart & Mass : Typical FMA demo. Attach a string to a wheeled cart. Run this string over a pulley at the edge of the desk. Not to worry, if you have no pulleys, just use the edge of the desk itself. From the other end of the string, hang a weight. [It's a nice idea here to hang an actual mass 'hanger'; a hooked thingee that will allow you to stack up weights so you can change the hanging mass. Devise something that works for you and your equipment. A simple large paperclip and washers will work in a pinch. The string should be just as long as the table is long (or side). Two demos to do: