First get some special case for the pen so it will survive any of the scenarios.

1. Bury it very deep underground
2. Eat it
3. Get it transplanted between your organs.
4. Move to china and get a new identity
5. Put it into the sword handle of your decorational sword on your wall
6. Shoot it into orbit.
7. Get a save at a bank.
8. Build your own castle to defend the pen.
9. Paint the pen in a different color. No one will notice it is the same pen.
10. Make it invisible
11. Give it to your friend you really trust.
12. Hide the pen for 30 years and then give it to the child Albert Einstein so he can come up with his results 20 years early.
13. Send it to Albert Einsteins mother, so she can protect it.
14. Throw it into the ocean and hope the tides will bring it back 50 years later
15. Put it between your couch cushions.
16. Put it inside one of your couch cushions.
17. Lend it to a museum for 50 years and tell them about the importance of the pen. No one will expect that you put the pen on display for everyone.
18. Give the pen to Maxwell, so he can come up with relativity instead.
19. Tell your son to come up with a solution. You fear you won't be there in 50 years to protect the pen.
20. Hide it in an old tree.
21. Bring the pen to the Vatican and ask the pope to hide the pen for you in some special save?
22. Glue it to the ceiling of your appartment.
23. Hide the pen inside an art gallery with the Text: "This is not a pen" written under the display.
24. Put it into your pocket
25. Give it to a pen collector to hide it between his other pens.
26. Try really hard to remember what you learned about physics in the future and come up with the Einsteins work instead.
27. put it into a chest and tell someone that it contains something really important.
28. Send it to your parents.
29. Examine it very closely. Maybe you are able to transfer its power to other pens.
30. Become really good at making pens, so you can give Albert Einstein another pen that is just as good.
31. Hide it in a mine
32. Found a cult around the pen that will be able to protect it against those evil forces.
33. Go to iceland and hide it in the snow.
34. Go to the Sahara and hide it in the sand.
35. Hide it inside of a book you don't like.
36. get a really high tower and hide it on the roof. no one will dare to go there
37. Hide it inside of some indiana Jones Temple
38. It's 1855? Put it under your hat.
39. Hide it on a yet undiscovered Island.
40. Put it so deep under water that it can only be retrieved with technology after 1900
41. Put it into a secret pocket of your jacket.
42. Give it to powerful authorities of your country of residence
43. Give it to a construction worker to hide it in a wall.
44. Put it into the Lascaux cave. It won't be discovered for more then 80 years.
45. Close your Eyes so you can't see it.
46. throw it real high and hope it does only come back in 50 years.
47. Put it into your wallet
48. Invent the Radio and send a signal to aliens to take the pen on there spaceship.
49. Hide a fake copy of the pen in an obvious hiding place and keep the real one always with you.
50. Make 50 copies of the pen and hide all of them in seperate locations. Only you know where you hid the original one.

1. rocket
2. slingshot
3. gun
4. trampoline
5. giant swing
6. giant vacuum pipe (I believe to have read about something like that on xkcd to bring water to mars)
7. throw it with your arms
8. "something" includes light, so you can send light to the moon using a small laser or a flashlight
9. build a tower/space elevator
10. grow a tree
11. use your magic wand
12. balloon
13. make a bridge out of legos
14. fly up there with your magic wings
15. get the moon to you -> get the moon out of its current orbit and crash it into the earth (or just get it closer)
    1. throw a big rock to get the moon out of its orbit
    2. use a giant laser to get the moon out of its orbit
    3. pull the moon toward the earth with a giant string
    4. Turn the earth into a giant magnet and pull the moon toward you.
    5. wait for the tides to pull the moon closer to you, then throw.
    6. frown at the moon hard enough to make it complacent then let it smash into earth
16. throw it through a wormhole
17. use a solar sail
18. trick yourself into believing you made it to the moon
19. rebuild the moon on earth
20. smash a giant meteor into the earth, so that some debris reaches the moon
21. pay Nasa/SpaceX to brainstorm for you
22. use a giant hook
23. put a small stone on top of an atomic bomb
24. make an icecream tower
25. freeze the seas and make a giant jumping hill
26. water a plant and let it grow until it grows toward the moon
27. genetically engineer a frog to jump to the moon from mount Everest.
28. throw many tomatoes hard enough to accelerate yourself toward the moon
29. build a giant ice bridge toward the moon
30. frown at the moon hard enough to ma
31. tell your hamster about it
32. find the strongest human you can find and let him throw a juggling ball
33. use a pencil as a catapult
34. use something with negative mass to launch it to the moon
35. wait for future generations/superintelligence to find a cheap way to do it for you
36. build a particle accelerator with the radius of the earth with a small hole to throw particles to the moon
37. wait for the neutrinos of your nuclear fusion reactor to reach the moon
38. become the leader of your country and start a space program
39. turn mount Everest into jelly and throw something on it
40. build a giant rollercoaster
41. make a tornado/typhoon/some other big wind thingy
42. put it into a volcano
43. use an enormous ventilator
44. travel back in time when the moon and the earth were still one and put your thing at the right spot so it will be at the moon in the present
45. use venus or mars as a gravity slingshot