This is an idea I got last year for a mathematical bracelet. Enjoy. You can decorate it with other bricks.

Materials:
Legos: 1 2/4, several 1/4, and some technic pieces
2 belts of different colors
Gorilla glue
hot glue
Gorilla super glue

Step 1: Glue and Cut the Belts

Pick your belts and glue them together with hot glue, leave a slot for the clasp on one side just long enough for the clasp which you'll add later. Wrap the peice around your wrist and cut it just about to the right length leaving some slack.

Step 2: Make the Clasp and Attach It

take 2 technic pieces and one of and a connecter as shown below. Length of the pieces depend on your wrist size and bracelet width. Attach these pieces with super glue. Then take this clasp and insert it into the slot we left in the belts. Pump the area full of hot glue and wait till it dries. I added a little bit of hot glue to help keep its hold.

Step 3: Glue on the Other Part of the Clasp and the Lego Bricks

Now glue on the other part of the clasp which is the blue one stud piece with the hole in it.(the hole should be round not plus shaped) Find the right spot on you wrist by putting it on and slipping the hole piece through the other clasp piece and making a mark on the bracelet of where to put it. Fill the inside slightly with gorilla glue and put super glue on the edges. I changed the look of the clasp to make it work better by adding a 2/1 with the one stud on top. To add the bricks do the same as the blue part of the clasp. Add a closure piece by attaching a piece with a plus shaped hole to the technic piece.(when I say attach a piece I don't mean for you to glue it on. Because you want to be able to take it off)