Ordered chronologically. In retrospect, I've assumed some pretty weak evil forces here, and mostly gone for variations on a needle-in-a-haystack type theme. 

1. Surgically implant the pen in myself
2. Theseus-duplicate the pen, give one duplicate to the evil forces so they leave me alone
3. Make a second, cooler-looking pen, ‘accidentally’ lose it to evil forces
4. Cut a hole in a dictionary/similarly boringish book, put pen in there
5. bury the pen somewhere random
6. carve a hole in a tree, put pen in, wait for tree to regrow over it
7. bury the pen, in a construction site, so it ends up underneath a building (harder to get to)
8. Hide the pen in the shaft of another, bigger pen
9. Sew the pen into the lining of a bag
10. Same but hem instead of lining
11. Make a box/piece of furniture, hide pen in the wood itself
12. Take pen apart, hide each bit somewhere different, reassemble as much as I can keep
13. Ingratiate myself with evil forces, ‘recover’ pen for them, betray them after the fifty years
14. Hand pen over to evil forces, steal back after 49 years (embed some tracking method in pen that will work in 1905)
15. Befriend a winemaker, hide pen in corked bottle of wine, keep wine for a long time ‘as an investment’
16. Hide pen in someone’s grave
17. Hide pen in someone’s grave, but by hiding it in their body and then waiting for them to die. Hope autopsies aren’t a thing yet.
18. Take pen somewhere remote, pay someone there to keep it for you
19. Keep a gun with you ‘to defend against evil forces’. Hide pen in barrel. Hope you never have to actually use the gun.
20. Convince art collectors the pen is valuable, sell to the highest bidder. Rely on whatever security they use for their collections. Fifty years later, buy the pen back.
21. Inscribe something on the inside of the barrel (so you can identify it), find as many identical pens as possible, hide them all. Good luck finding the right one in fifty years.
22. Hide at the bottom of a lake, hope lake doesn’t silt over/get dredged
23. Use the pen like you would with any other pen. Evil forces assume you’d be hiding the real pen, so that can’t be it.
24. Taxidermy
25. Become an expert in art conservation. Re-frame an old, valuable painting (hide pen in frame). Fifty years later, re-frame it again.
26. Hide pen in the walls of a new building. Demolish after fifty years.
27. Take one part off pen. Destroy the rest. Attach remaining part to a different pen after fifty years. Claim that if the ship-of-theseus thing before counted, this does too.
28. Offer pen as collateral for a huge, long-term loan.
29. Or just open a bank account and put it in your safe-deposit box, I guess.
30. Convince a member of evil forces to turn their coat. Give pen to them.
31. Hide pen in fake brick. Make building out of that plus other real bricks. Remember which brick it was, demolish building later.
32. Bury pen, plant a tree over the top (so that hopefully its root system surrounds it).
33. Give pen to nomadic group for safe-keeping, hope evil forces can’t find them.
34. Sell to pawn shop, along with as many other pens as you can get your hands on. Hope evil forces don’t know what kind of pen they’re looking for, or which of the many pawn shops you sold pens to has the right one. Inconspicuously buy back after a while.
35. Hide pen in walking stick, horribly cripple yourself, claim you just have the walking stick for the obvious reason.
36. Actually, just hide the pen in someone else’s walking stick instead.
37. Hide pen in an umbrella handle.
38. Give pen to church, claim it was, idk, used to sign Jesus’ death warrant. Have them hold on to it as a relic.
39. Remove ink from pen. Leave in the stationery cupboard at a university. Assume no-one will want it, but no-one will bother to throw it out.
40. Choose someone at random, give them the pen, tell them to keep it. Make sure evil forces don’t find out who you picked.
41. Same, but hide it under the floorboards of a random house, without its occupants knowing.
42. Using future-telling powers, give to a random future classmate of Einstein’s. Tell them to give it to Einstein when they meet him.
43. Leave pen in the stationery cupboard of the relevant patent office (again without ink if necessary).
44. Use future telling powers to work out how you’ll manage to hide the pen. Do that.
45. Decide it doesn’t matter which exact pen Einstein uses. Go into the pen-selling industry anyway. Sell him a different pen when the time comes.
46. Throw into a river, which probably then silts up and then changes course. Dig up from where the river used to be fifty years later.
47. Bury pen in an area high in some mineral people might want in fifty year’s time but don’t at the moment (silicon? uranium?). Get in to the mining industry.
48. Drop the pen in the ocean near the Netherlands. Dig up a tulip farm fifty years later.
49. Leave the pen in the dead sea for a while, until a huge salt crystal forms around it. Donate to a museum as an interesting geological specimen.
50. Bury the pen in an already-discovered archaeological site. (Become an archaeologist). Decide there was more to that area than was initially discovered, and dig it up further fifty years later.