Play Zed, put in a stationary Vayne bot that only uses Condemn, bring Condemn CD to 1 second, bring Zed ult CD to 1 second, and keep practicing how to ult her condemn.
Remove the internal cooldown from Yasuo's E, bring Flash cooldown to 1 second, and practice dashing through jungle walls with jungle monsters, then flashing back over the wall to keep practicing the wall dash.
Your turn.
warpenguin55517 votesi would just put a stationary
or
down and just kill them over, and over, and over again. Also id try late game builds like what another guy said
Desiderium1 votesNot really a productive use of time, but definitely a cathartic one.
Bârd13 votes/set_movementspeed_MasterYi_1000000000 /dance
Seriously though? /set_cooldown_TemperedFate_0 then practice using it to block incoming spells and landing it globally.
HayRoss11 votes
Put 0 cooldown on the barrels, cover the ENTIRE map with barrels, buy 6 infinity edges, spawn in as many Sona bots as possible, detonate
CENTAKILL!
Kyoukocola10 votes2 who have no cooldown on W and eat as many oranges to see which is the most k.
But seriously, I wanna practice landing my Nami Q on really fast targets or Threshes Lantern.
BluePolarizer9 votesDodging Malphite ults with Alpha Strike.
Seeing who is the real champ: Safe Vayne vs. 40% CDR Jax.
Ekko W timing.
Sypher The Poro9 votesFirst thing I would do? Get a bunch of friends. /set_cooldown_Reduction_100%
Because why the hell not.
Jbels8 votesPermanently set myself to level one, disable all of my abilities, keep all of my base stats at their base no runes or masteries evel, and just practice last hitting
Desiderium1 votesThis actually wouldn't be a good thing to practice, especially if you play a lot of AD champs.
Having extra AD to be able to secure more last hits is one of the advantages of playing an AD champ.
windwolf7771 votesWhy is that a bad thing? It gets him ready if he accidentally brings the wrong rune page, it gets him to know how a champ feels at its weakest, and sounds fun. I don't see anything wrong with it
MaximumNinja177 votesPut rengars cooldown on 10 and practice his ult oneshot combo .
Get some friends and play minesweeper with a 1 sec ult rechage time on teemo
Just derp around in general with funny champs
imagine the yorick ghouls
windwolf7771 votesa bunch of level one 
vs 
Plants vs Zombies :D
Daedalus8716 votesMostly practice different champion's jungling capabilities and different build paths.
Keevalroy5 votesPractice Charming or Cocooning during a Cataclysm cast or Regnar bush jump.
Zone of Endless1 votesPractice Charming or Cocooning during a Cataclysm cast or Regnar bush jump.
J4 immune to cc during ult casting time
Keevalroy1 votesI've charmed him out of the cast, I've seen thresh interrupt the cast with Flay, I've also seen Elise Cocoon him out of it. It's possible if you time it right. It's not like Unstoppable force.
Big Lincoln5 votesI would put as many Teemos as I need to go from one end of the map to other
And then fucking 
dunk them all
Photos of Ghosts5 votesWardhopping to dodge CC with Kata, by placing wards that last 0.25 secs.
AMYS GRAVE4 votesTest late game builds on champs in under half an hour.
Cowgirl Ed4 votesrepeat_infinity( /destroy_champ_Vayne /resurrect_champ_Vayne )
Dr Poro2 voteshttp://www.lolking.net/models?champion=67&skin=0
Go to animations and choose "Death". Sorry, no sounds, but it's worth something.
ForgottenLuck3 votesI would practice dodging skill shots by putting 4 nid's in a game that could only use q
Westonbirt3 votesAlistar with 1 second global headbutt.
DemaciaClown3 votes/set_champion_sion_ultimateCooldown_0
Then I would spawn a alistar spamming wards and be like COWW WAAARDSS!!
Wind Whistler1 votes/set_champion_sion_ultimateCooldown_0
Then I would spawn a alistar spamming wards and be like COWW WAAARDSS!!
Priceless.
DemaciaClown1 votes10 /10
Hex Veiler3 votes6 minions per wave --> 600 minions per wave turret health * 6 /set_AD_Sion_1000000000
Ult down mid lane
stabby himiko1 votesCrash the game before you kill half of them
Hayaishi22 votesflash malphite ults
Desiderium2 votesDoesn't need much practice IMO
Really easy to do from max range
Snake Town2 votesMake some weird game modes with friends
Saianna2 votes/set_truestealth_Eve_true
For the feels of good ol' times
TASTE THE SAND2 votesi would perfect my ad soraka build
Desiderium2 votes"Perfect"
"AD Soraka"
Pick one
IcyPepper5 votesCan't have one without the other
L Psy Kongroo2 votesPractice dodging abilities with Shaco's ult.
TurquoiseYoshi2 votesURF.
That is all.
Lazypeon1002 votesTest for bugs. Sometimes while just playing I'll notice a bug (sometimes game breaking) and I say to myself that I should totally try to recreate it so I can report it to riot. But then I remember I need to get someone else usually to help me, enter a custom game, and spend a long time buying items and what not to try to recreate it.
An example, awhile back I lost a ranked game that my team was winning. We had both a quinn and a Yorick in the game. Team fight broke out, quinn was in bird form and died, came back thanks to Yorick's ult, and proceeded to time out til her death after the fight.
When she repawned, she was permanently stuck in bird form. She had movespeed like she was still quinn and not Valor, her passive didn't work, and she couldn't use her ult again.
I would love a sandbox mode to be able to more easily recreate that scenario so I can record it and show it to riot so they can fix it.
Jofeyy2 votesSet up a dummy champ and practice 
and 
insecs with ult.
Aych Pii2 votesPractice my full burst combo on Jayce.
(From hammer form) R>AA>E>Q>W>R>Q>W>AA>AA>AA>E>D(ignite)
Yormaughm1 votesUse math to prove which champions with which items can literally completely shut down other champions mathematically 100% of the time so that we can virtually, if not actually delete half the champions in the game.
J/K I wouldn't do this, but someone would. And it would ruin the game.
Dr Poro3 votesMaths = theory. You don't need a sandbox mode for that. Sandbox is for testing things out, practice, empiric. For finding out the mathematically best option, you'd only have to try out everything once to see how it interacts or even not at all and just look at the code (in theory).
You won't come up with a new theory because of sandbox mode, but you can become incredibly good at certain mechanics.
Blade of Justic1 votesMake Kalista invincible and see how spears mixed with her E it takes to get to the Tooties Roll center of a Baron.
PDE5 Inhibitors1 votesMake Kalista invincible
So, no changes?
LANWhale1 votesI would block the minions with a whole bunch of Zz'rot portals and then release them. Cuz ya know, reasons.
Desiderium1 votesYou could just spawn more minions instead
LANWhale3 votesThat's what they would want you to do.
Shroom Junkie1 votesI would make URF mode.
gib pls.
Desiderium1 votesPlay Bard and practice the exact locations where I can maximize the distance of Magical Journey through walls.
JustMyBassCannon1 votesRemove the cooldown of Kindred's Mark, get to 80 stacks, laugh at 100% current HP physical damage.