What can I do at home in the summer (10 points)?

Okay, so I need some very fun ideas for being at home in the summer. I can't go out and I don't have a lot of stuff to use to entertain myself. My parents are not home they're always working. I'm home with my grandmother (she doesn't know how to drive) and my little sister (she's 4). I'm pretty bored. I'm a computer addict but I'm getting pretty bored of it. I am an outdoorsy person just not in my backyard. My backyard is pretty cramped up. I can't invite friends over and can't go over to a friends house. Please help! Thanks in advance. :)