When I prepare to run a TTRPG I always prep some random Hazards and Rewards
I have been using the Hazard System, by Necropraxis Gaming, for
spice and random encounters in my games. I find it is simple and
quick to develop ideas on the fly. It provides variety, especially
when used as a compound table, but doesn’t get overwhelming. In
short the system is this: Roll 1d6 per round. Rounds can be
any time frame, simply adjust the chart to fit. The d6 tells you what
kind of event occurs; 1) encounter 2) fatigue 3) expiration 4)
local 5) roll again and give hint 6) free turn. I find that coming up with two
or three ideas for each category and that are specific to the
adventure is quick and very helpful. Find more examples here.
The last section of information that I usually prep are rewards. I
list here important things that players might find, random tables
of loot. I highlight any items that are necessary to the story or
specific to a particular PC. For the most part though I use random
tables for loot beyond what is written into the adventure. I like
to use DonJon’s website for random lists of things that fit 5e.
I also like to add OSR style magic loot to my games. IMO Old School
Revival loot is any loot that is bizarre or unusual and has a
particular use case. Like a Silver bird cage that slays whatever
is placed behind its bars, slowly, over one long night. That which
is killed reanimates twice as strong, as a raging uncontrollable
undead (ty Mork Borg). Or perhaps a Limb Seed: Food, 1 use, Apply
to a severed limb or stump to grow a new limb in 1D4 days (ty
ICRPG).