There are many different pranks to play on a little brother. Common pranks include putting plastic wrap on a toilet, stuffing his shoes or convincing him he is late for school on a Saturday.
Fun Stuff To Do provides a list of pranks to play on people, some of which are relevant to a little brother. The most successful pranks create a realistic and believable setting: for example, changing the calendar in his room to make it look like it is a different day of the week, or getting dressed in advance to make it look like others are ready to go to school.
Some other ideas for pranks include gluing his shampoo bottle shut, freezing his cereal, putting syrup in the liquid soap, putting shaving cream on his face while he's sleeping or sewing the fly shut in his underwear.