Jump on the flagpole at the end of a level with the last two digits of your time 
being the same number, and the number of fireworks that appear will correspond to
the number of the digits. For example, "22" will result in two fireworks, while 
"77" will show seven fireworks, etc. Additionally, certain numbers will also give
you a bonus item, which are as follows:

Fire Flower: 33 or 34
Ice Flower: 55
Star: 88 or 99
Super Acorn: 77
Super Mushroom: 11, 22, or 66