Tub and sink stoppers can be pretty standard, but with a few items and a little work, bath time can be loads more fun!

Step 1: You'll Need. . .

7" x 9" piece of vinyl cloth or something similar - you'll want something fairly heavy and waterproof (fabric made from fused plastic bags or duct tape would likely work well if you didn't want to purchase other fabric)
A small grommet and a grommet tool
A tub or sink stopper
A length of thin chain approximately as deep as your tub to the point at which you'd fill it for a bath - 10" or so should be more than enough
A sewing machine (you could sew it by hand, but a sewing machine speeds up the process and ensures a strong stitch)
Pliers (optional but suggested - I used needle nosed)
Hole punch or leather punch (optional but suggested - I used a leather punch that came with my grommet tool)
2 pennies (or other small thin weights) and waterproof glue (optional - I didn't use weights but you could to ensure that the bottom of the boat stays down in the water)

Step 2: Fold It Up and Sail Away

If using weights, apply a small drop of glue and affix the pennies to the bottom side of each of the short ends of the fabric
With your fabric in front of you vertically so that the pennies are against the table, fold the top half down
Fold in the top corners to the center
Fold the flaps on the front and back of the bottom up
Open the triangle that results and flatten it to form a diamond shape
Fold the lower triangle of the front and back up to meet the top corner
Insert your thumbs into the pocket that is created at the base of this triangle and open it, flattening it down into a diamond shape
Holding the center inner triangle, pull the sides down to form your finished boat
Pin and sew the sides of the boat to ensure the whole thing holds together

This video (in addition to the photos) should help to illustrate the folding process - there are also these great instructions on the folding technique: