You can gain infinite height in Metro Kingdom by abusing the Hat object. (This object is on a bench east of Pauline after achieving peace. It disappears once you collect moon No.57) You need to jump and immediately throw the Hat with Y, then dive towards the Hat and on the frame you bounce off of the Hat, press Y to pick it up followed by a second Y press to throw the Hat again and repeat the process. The Y press to pick up the Hat when bouncing on it is difficult and only has a window of about 2 frames.

In 2-player mode; if you make both of the Chargin' Chuck captures face each other and closely positioned, you can gain infinite height by simply shaking the Cappy controller (or by repeatedly pressing the Cappy flight button). This happens because when Chargin' Chuck is released from Mario's possession, he gains a slight height boost. 2-player mode allows you to repeatedly capture between both of the Chargin' Chucks with ease. Discovered by Syrkl

Interrupting the Dogs Jump-Animation by bouncing off of the Dog itself will freeze the Dog in mid-air whilst the Dog plays out its Hit-Animation. After this animation ends the Dog will re-do its Jump-Animation at the point it froze in air thus gaining height. You can interrupt this second jump and all consequential jumps to gain infinite height. To do so you must wait a certain amount of time in-between bouncing off of the Dog. A good rhythm is to; - Hold B or A when bouncing on the Dog to do a large bounce - Press Y or X to throw Cappy at the apex of the bounce - Press ZL or ZR to perform a groundpound shortly after This rhythm is seen at the end of the video linked.