I wonder if Magic Cards (Specifically the Power Nine cards and Beta Dual Lands) are not a good investment? They have multiplied by about 10 in price over the last ten years. I've known people who had 300 Serra Angels, a terrible investment whose price decreased from 8 dollars to about 1 or less over the years.
Magic is both a collectible and a game, I don't know how that factors in expected value return.
Usually the top 0,3% (in future relative scarcity in Type1 and Legacy) increase steadily in price no matter what, top 1% unless they are reprinted (in which case both up and down can happen) and most of the rest goes down. But only with years and years of experience can a player tell whether a card will belong to the select few. Svi may have informed opinions on that.
Just to spend some time calibrating future-me confidence in Magic price calibration, I'll say some outrageous hypothesis: Up: Time Vault, Mana Drain, P9 except timetwister, FOW, Karakas, Fetch, Duals, mutavault, moxen. Down: All non-tribal creatures pre-2008, jace, all dual trual lands except above, baneslayer, wrath of god. There you go future 2017 me, stop trusting yourself that much and never invest in what you mind thinks it is superexpert at without much evidence.
I wouldn't know about magic cards, sorry.
Stamps would be my choice because they have many advantages over other types of collectibles. Magic cards may share many of the benefits stamps have over other collectibles considering the similar format, but stamps surely have special perks magic cards don't.
Stamps have the advantage, that they are the number 1 collectible in Germany and many other parts of Europe and they have been forever. Coins and other things don't come close in terms of how widely they are collected and the bigger the demand, the easier it is...
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.