Just to add to the previous post.

Missions - yes, you can do the simple math to figure which mission gives more canes per hour. But shorter missions also have another advantage - with them, you get to re-roll more often so you have a better chance of rolling a 24hr epic, 8hr legendary, or 24hr legendary - these are the missions that also give multipliers, IIRC.

In other words, a 30min common mission might not be any better (in canes per minute) as some 4hr mission of higher rarity, but by choosing it you are not stuck with one mediocre mission - instead you get to re-roll 8 times in the 4 hour interval, with a chance of getting a really good one.

Gems - yes, there is a decoration that increases key drop. But keys are limited to 200 per day and if you use an auto-clicker then you will get them all easily even without that decoration (and I cannot imagine playing this game without an auto-clicker).

And yea, I usually get 0, 10, or 20 gems from the daily 200 keys; more on a lucky day. Tournaments are also a significant source of gems, as well as achievements (early in the game).