CTR glitch thing

Depending on which save you load, you'll have the save's items unlocked after the setup. Having 4 keys means you can enter Oxide etc.
Doing this without loading a save file will just give you nothing, it's fresh.

Setup:

1. Pick Battle, 2P, whatever characters at all it doesn't matter. What you put on P1 goes to the hub world though.
2. Go to Rocky Road, turn on Life Limit Mode
3. Put P1 to the yellow slot and P2 to the blue one
4. Might wanna pick triple missiles to make this quick, it's optional though do whatever
5. Kill P1 and proceed to Change Setup
6. Start Battle again, kill P1 again and Change Setup again
7. Start Battle again to warp to hub 5

From here you can enter Relic Races and boss battles. The game will still treat races like it's in Battle mode so don't expect to just pause menu your way back to the hub.
If you go to Oxide you can either complete the race for fun or just straight-up quit from it through the pause menu. Afterwards just get a fresh start going with Adventure mode and enter Cove for a "boss race" against Crash. Winning it ends the game.

In conclusion:

Whatever boss race you entered after the setup will be the one you're beating through Cove afterwards. This means you need a pre-made file that can enter Oxide to actually beat the game in a single race.
With a fresh start you can play up until Ripper Roo, go do the setup, initiate the race and beat it in Cove, BUT it'll trigger Roo's beaten cutscene and give you a key on top of the 0 trophies you have on your fresh start, meaning you'd need to beat hub 1 again. You can't dupe the key though, as it recognizes you've beaten Roo after you're done collecting the trophies.
So in a real run, you'd play up until you're done with Pinstripe, standing in front of Oxide. Go do the whole setup and laugh as you're losing a solid minute for no apparent reason.