Falling bodies
If you can search a building that is about 20 m high in your locality you can examine how gravity makes bodies fall faster the longer it acts on them. Get a piece of string long sufficient to reach from a point at least 20 m high to the ground. Fasten the cord so that it forms a straight vertical line. Opposite a window 20 m from the ground tie a piece of coloured cloth or yarn to the string. At about 5 m below this point tie another piece of coloured yarn.