Farm gadgets

These chests are actually impossible to open as 2B.

However, after completing the story the first time and receiving ending A, replaying the game will have you play as 9S. 9S's signature ability to hack machines comes in handy, as he can use this ability to hack locked chests that were unable to be opened while playing as 2B.

During your first playthrough, since you can't do anything with these chests, just make a quick note of where you saw them and move on.

It's worth noting that this ability opens up a lot of previously unavailable side-quests, as well, in case you felt that there was a serious lack of side-quests your first time through.

I believe I've found the best possible way to farm levels in this game. As mentioned in this thread, the giant golden bunny statue at the front of the Amusement Park is actually a level 80 enemy.

Pleasantly surprised to find out that big machine life-form statue with the horn and bunny ears in the amusement park is actually a living enemy. [...] It's a level 80 enemy and it's really only feasible to defeat it by hacking it. Which you want to do because each successful hack on it netted about 11,000 exp. 2 hacks + it dying got me like 50,000 exp (slightly amp'd by my Exp +5 plugin chip). Went from level 66 to level 70 from one easy foe.

You may have to attack it for a few minutes before it notices you and starts moving (you won't even see a health bar before then). Once it starts moving, the easiest way to kill it is by hacking it as 9S.

When I just tried this with an EXP Gain +6 chip equipped, I received 13,000+ EXP for the first hack and 14,000+ EXP for the second, then a ridiculous ~50,000 EXP for defeating it. I leveled from 69 to 74 in roughly 4 minutes.

Here's a video of the entire thing.

Side note: while waiting for the video to upload, I defeated it a second time and boosted to level 77.

Update: It took less than an hour total to boost from level 69 to 99 using this method.