Annoying your family during boring summer vacations.
If you annoy everyone enough, maybe next time they'll let you stay at home, or at the very least, beat the shit out of you for being such a brat so that you are in the hospital for their next vacation.
For all those who don't know, siblings are brothers/sisters.

After everyone is done packing, find their suitcases/backpacks and remove all their underwear and socks.

Buy a notebook and keep detailed entries about the day, making sure to write a lot about how ugly everyone around is and draw sketches. Leave the notebook out and when someone reads it and complains, throw a fit since they invaded your privacy. Make sure to draw unflattering pictures of everyone.

Dig a hole in the sand and hide in it. Hopefully your family will forget you and you can then live the live you always wanted at the bottom of the hole.

Wear heavy perfume/cologne so none of your family members wants to be near you.

Get a pointy stick and jab at everyone and everything with it.

If at a campground, round up all the stupid looking people and start your own cult. Not only does this kill some time, but it provides you with some people to do your evil bidding for a while.

Take a small pin and put a few holes in all of the inflatable water toys.

Find an ant hill and train them to obey your commands.

Climb a tree and then scream to be helped down because you're too scared to come back down yourself. Make someone rescue you. Repeat this every other day.

Start piling twigs and branches in the corner of the porch or something, claiming that you're building a nest like the birds you saw on TV last week.

Attempt to learn another language using a self-help book, asking everyone around you how they think each word should be pronounced.

Be the first person to wake up and take an extremely stinky shit each day, so that everyone gets to wake up and wander into the stink.

Refuse to go anywhere unless there are ice cream treats involved.

Whistle off-key all day long.

Treat everyone like they're your personal bodyguards and servants. Tell store clerks and such that they are just there to look like they're your family but you can't say why.

Keep detailed notes on everything that a family member does each day until they threaten or actually beat you to make you stop.

Pretend to have nightmares and wake up screaming each night.

Collect bugs and spiders to put in sibling suitcases and beds.

Eat whatever sorts of food makes you fart a lot.

While at a cottage, be the first person each day to take a long, extremely hot shower so that there is no hot water for anyone else when they have their showers.

Hide all the good snack food so no one but you gets to eat it.

Refuse to give anything but a blank look or horribly forced smile in photos.

Throw things randomly out the car window while traveling.

Insist that your friend the garden gnome must come along with you everywhere you go.

Set aside your own towel, and with everyone else's, rub them all over your sweaty ass.

Create threatening letters like "Something of yours has been dipped in the toilet." and leave them on the pillows of your siblings.

In the sand, create what looks like each of your family members and then kick the heads in while laughing uncontrollably. (Create the sand bodies laying on the ground, it's easier.)

When no one is looking, piss on the sand castles/creations by your family/siblings.

When food is made for you, eat so much of it that you cause yourself to throw up all over the place. Then claim that the person who made the food was trying to kill you.

Take pictures of bizarre things and random people with the family camera. Do this when they're not looking, so when they develop the film, they find the random pictures.

Throw bread, popcorn and other items for birds to eat on the top of the car/truck. Birds will come and eat it all, shitting all over the vehicle.

Swim near someone and piss, then swim away.

If swimsuits are hanging in the bathroom to dry out, each time you're in the bathroom, run them under the tap and wring it out so that they are always damp. You can do this with towels as well.

Water down the sunblock, or put some type of perfume in it so all the bugs are attracted to whomever puts it on.

Dump out most of the bugspray and fill it with sugar water. (Keep some of the bug spray in there to retain the smell of it.)

If you have leathal foot or body odor, make sure that you sweat and stink profusely.

Wake up at the crack of dawn and sing your national anthem. If anyone complains, accuse them of being terrorists.

Insist on reading books outloud, but not in such a way that people can sit and hear the whole story.

Rant. Do nothing but rant on about everything wrong around you. Write furiously in notebooks, and rant to people on the street. Take pictures of the things that you're ranting about.

Be a backseat driver. Point out signs, things at the side of the road and everything else until they duct tape your mouth shut or strap you to the roof rack.

Replace all the sunblock with sour cream.

Rub all the towels with used kitty litter.

Insist that you are learning another language and practise speaking it the whole way to wherever you're going.

Paint yourself with body paint and insist it counts as clothing.

If forced to 'shop' with your family, point out the ugliest stuff possible and suggest that your mom/dad buy it as it would look 'really cool' on them. See if you can get them to believe that florescent colours and sequins are making a comeback.

Disappear for a few hours and when returning, tell everyone you were looking for yourself and after getting bored with that, started up your own cult. Actually start up your own cult, and have the members bring you gifts and food each morning. Make sure they follow you around and do your bidding. If anyone questions you, have your cult chant at them.

Try to train birds to attack on your family on command.

Get a glass of water (or lemonaid) and sprinkle it on your brothers/sisters clothing so that all of their clothes feel damp. If you use lemonaid or any other sugared drink, bugs will be attracted to them.