excellent idea!

yes, gravity is not something tangible. i have chose feeling of tactile sense of my feet on the ground. so if i am awake, i can feel it and if i am dreaming, i can't feel the ground very clearly. i have practiced this method since 3 days ago and it is very very easier than ADA and it is not taxing and feels good. even i can have this mindset while studying.

other choices have been well introduced by Ctharlhie.

i copy and paste his choices from his topic to here

1. Gravity
2. The feeling of clothing
3. Temperature
4. Feet on the floor
5. Ambient noise
6. Light sources
7. Visual snow/floaters
8. Tinnitus
9. Air Pressure (wind, breezes, droughts)
10. A sense of the breath

i think number 2, 4 and 10 are easier for most people.