Hidden Hearts:

If you're running low on lifeforce, get out your
vaccuum and start cleaning lamps and vases. Almost every second vase has a heart hidden inside and nearly every lamp or chandelier either yields money or health.

Secret Warp Zone:

Running around from one end of the mansion to the
other can be a very time consuming task. Thankfully, Nintendo has implemented hidden warps that let you zoom back to the entrance hall.

Gym Room Warp:

In the gym (the room with the weights and treadmills), turn towards the mirror at the back wall. Switch to your Game Boy Horror's first person mode and examine the mirror with A. After a really trippy warp sequence, you will find yourself in the entrance hall of Luigi's Mansion