It's perfectly OK to play safe pranks on friends, especially if your friends will find them funny and not malicious. A prank is considered safe if it doesn't hurt anyone, physically or emotionally, it doesn't damage property, and, of course, it is not illegal.
Most pranks to play take at least a little bit of planning so that you can prepare them to work the way you want them to. You will also want to prevent your friends from finding out ahead of time and spoiling the surprise. Here are some easy ones that take less than an hour to set up:
If one of your friends falls asleep in the car, on an overnight trip or during a sleepover, draw a mustache on his or her face and then take a picture of the sleeping victim. Make sure you use a water soluble marker. Do not use anything that says permanent on it, since this type of prank crosses the line from fun to potentially damaging (and certainly mean-spirited).
Fake Pest Prank
Put a fake spider, snake or other pest in a friend's lunch bag, backpack, briefcase, etc. The person will be in for quite a shock when opening the item to retrieve something. Make sure the pest doesn't have sharp edges so the person doesn't risk a cut. If the proposed victim is deathly afraid of certain types of pests, it may be a good idea to skip the prank and choose someone else. Someone who has a phobia for creepy-crawlies won't appreciate the joke.
Wet Paint Prank
Put a paper sign that says "Wet Paint" on a painted chair or park bench that is actually dry. When your friend walks by, sit on the item as if you didn't notice the sign was there. If your friend laughs and points out the sign or refuses to sit down, this is the time to point out that the joke is really on him or her. Removing a real "Wet Paint" sign from a freshly-painted object is not a fun trick to pull, though, since it could lead to someone ruining their clothing.
Fill someone's bedroom, dorm room, office or car with balloons. Recruit some other friends to help you or have them inflated at a party store. If you try to do this yourself, you may end up with quite a headache from blowing up multiple balloons.
Get an entire box of plastic forks and stick them tine-down on your friend's lawn. You can even place them in the shape of a word, such as "gotcha." Make sure you don't push the forks down so far that they damage the grass. It'll be funny watching your friend pull them out one by one.
Have someone who knows your friend give you access to her office or bedroom while she's away. Rotate all of the furniture about 15 degrees. At first, your friend might not know what's different, but she'll figure it out when she sits or lies down in an unfamiliar spot.
Plastic Wrap Prank
Cover a doorway with plastic wrap or clear plastic sheeting. Let your friend run into the wrap when trying to walk through. Make sure your friend doesn't approach it at a run, and help peel the plastic away from his nose and mouth immediately so there is no risk of suffocation. Do not play this prank on young children.
Determining How to Play Safe Pranks on Friends
Once you've decided what pranks you'd like to try, it's easy to play safe pranks on friends. First, gather your accomplices; then give everyone a task. It's good to have at least two people work on setting up the prank and one person serve as a lookout. That way your friend won't show up unexpectedly and spoil the future surprise.