Dress up your wheel spokes with colorful tape, ribbon, chenille sticks, or drinking straws (cut them lengthwise and place one over each spoke).  

Wrap streamers around the bars of your bike frame.  

Cut stars or other shapes from cardboard and tape them to your bike frame.  

Decorate a shoebox to make a basket and tie it onto the handlebars.  

Make a license plate from cardboard to hang off the back of your seat.  

Tie bells, bike horns, mini pinwheels, and streamers onto your handlebars.