All it takes is approximately 2-5 hours one day a week to be a mentor. You can make a difference in the life of a child, and will make a wonderful little friend, too.
http://www.mentoring.org This is a great site for finding a mentoring program in your area. These programs will match you with a child who has similar interests.
http://www.pbskids.org/zoom Zoom has lots of activities, recipes, games, and much more within its webpages. Although it's not a site specifically intended for mentors, you will find tons of great ideas for what you can do together.
http://www.mentoring.org/be_a_better_mentor/be_a_better_mentor.adp Mentoring.org also features online training for becoming a better mentor.
Activities to do with your little sister or little brother include:
Crafts (invest in a craft book that uses common household items).
Help with homework.
Play with pets.
Go to a museum.
Play an educational computer game.
Go to a park.
Run some errands.
Play musical instruments.
Build a cardboard dollhouse (use wrapping paper for the walls and carpet remnants ($1 at a carpet store) for flooring.
Build a pinewood derby car.
Get involved in a summer reading or activity club.
Shoot hoops or play H-O-R-S-E or P-I-G.
Go to a pet show, such as a cat or rabbit show together.
Visit a zoo.
Make little miniature movies or commercials together with a camcorder. Make a copy for the child to take home.
Take photographs and create memory scrapbooks of activities you do together.
Carve a pumpkin for Halloween.
Go sledding or build a snowman. Make a colored snowman by putting food coloring in a spray bottle.
Volunteer for a day at an animal shelter.
Decorate for the holidays.
Study a foreign language.
Camp in the backyard.
Plant seeds -- wildflowers or even vegetables -- to enjoy later on.