Step 1: Figuring Out What You Want to Put in the Package
You can make them feel self-conscious (I would only recommend to do to a guy because some girls wouldn't think it was funny) by putting self cleansing products such as deodorant, shampoo, mouthwash, ect...
You can make them feel happy and wonder why they got this by sending money to them, leaving them asking why they got it.
Freak them out by sending them their things that they lost (that you took from them). Do this with anything invaluable but that could only be theirs. And try to send a few things a couple of different days just to keep them guessing.
Any of these items will make the victim continue to guess who sent these things and why. All of them are fun, harmless jokes and remember that if they start really get scared that you should reveal yourself and reassure them that it was a joke and that you meant no harm by it.
Step 2: The Package
You can either hand deliver the package to the person and leave nothing on the package except for their name.
You can also send it through the mail with no return address and leave a name such as the Easter Bunny or Santa Clause or just no name at all. This one is my favorite.
If you know this person very well I would recommend typing everything so that they can not decipher your handwriting and trace it back to you, this would ruin the joke and you would reveal yourself.