I'm looking to take on a really, really, really enourmous chumra which will greatly affect my life to show how dedicated I am to Hashem. Does anyone have any suggestions?
ותהי להם הלבנה לאבן והחמר היה להם לחמר
The moon wailed to the Rock over them; and the donkey was a stringency for them.
This teaches us that the moon's complaint over its size was actually "over them", that is, for the Jews. For the moon knew that the Jews would be compared to it, and was therefore protesting its size. For this reason, the pasuk continues, "the donkey was a stringency for them": the stringency of riding on a donkey was adopted by the Jews to appease the moon. After all, the cow jumped over the moon but cannot be ridden with a donkey, so by riding a donkey we are showing a lack of desire to ride a cow, and thus showing honor to the moon.
Now, the custom of donkey-riding never gained much traction, though it was followed by such greats as Avraham and Moshe (and was, l'havdil, adopted by Bil'am in an effort to look good). It will be revived fully in the time of Mashiach, but until then it is certainly a worthy chumra to undertake.
This isn't really a chumra, but an actual halacha that nobody seems to keep anymore.
Whoever makes havdallah on any given Saturday night is required to buy (or pour) an alcoholic beverage for everyone that discharged their havdallah obligation through his recitation.
We even make reference to this obligation in the havdallah service itself:
La-yehudim hay'esa ora ve-simcha, ve-sason vikar, Kein ti'heyeh lanu Kos
"The Jews had radiance and happiness, joy and honor. Therefore, you will give us a cup"
Before you do a mitzvah -- any mitzvah at all -- don't just say a hineni memuchan umzuman -- that's for ordinary people.
Instead, you need a twenty-minute hineni memuchan umzuman, belted at the top of your lungs, that also adds ashreni mah chelki, hineni mekayem mitzvas asei yoser tov mikol bnai adam shebaolam... ("how fortunate am I, I shall hereby perform this mitzva better than anyone else in the world...") (spitting at this point is recommended -- all those other people! Ptoo Ptoo Ptoo!)
Well, if you're a fool, the Tosefta (Eduyot 2:3) recommends you follow the chumrot of both Bet Shammai and Bet Hillel. But the Tosefta warns that this is only for those on the appropriate level, so handle with discretion. As always, please CYLOR before relying on advice from strangers.