These things are just as easy to mail as it is to ship a standard package or crate. Just write the address clearly with a permanent marker or create a proper address label and adhere sufficient postage (dependent on how much the item weighs and where you are sending it to), and the USPS will literally mail it for you!
1. A Coconut

2. A Potato

3. A Box of Candy
A great idea for kids! As long as the box is less than 13 ounces, you can slap those stamps and address label on it, and send away. Hopefully your mailman doesn't have an excessive sweet tooth.
4. A Flip-Flop

5. A Sombrero
6. A Lime

7. A Rock
Hand-paint a design and a label directly onto it, stamp it, and ship it!
8. A Brick

9. An Inflated Beach Ball

10. A Piñata
11. A Box of Balloons
Slightly inflate water balloons with notes or money inside each one and fill a box or container with them. Won't weigh much to ship!
12. Supplies in a Bottle

13. A Rose in a Bottle

14. A Disposable Camera
Take it out of the box, snap a few fun pictures, put it back in the box it came in, tape back up, and send away!
15. Plastic Easter Eggs

16. A Frisbee

17. A Bucket and Shovel

18. A Seashell
Now you know what to do with that giant clam shell you found on the beach.
19. A Pillow

20. An Orange
With a permanent marker, write "ORANGE you glad ______."