Can I activate Multani, Yavimaya's Avatar on the battlefield?
You won't be able to active Multani on the battlefield. See Comprehensive Rules 113.6k:

113.6k An ability whose cost or effect specifies that it moves the object it's on out of a particular zone functions only in that zone, unless that ability's trigger condition, or a previous part of that ability's cost or effect, specifies that the object is put into that zone.
Example: Reassembling Skeleton says &quot;{1}{B}: Return Reassembling Skeleton from your graveyard to the battlefield tapped.&quot; A player may activate this ability only if Reassembling Skeleton is in his or her graveyard.

Multani states:

(1)(G), Return two lands you control to their owner's hand: Return Multani from your graveyard to your hand.

Since it says it is returning from your graveyard, you can't active unless it is in your graveyard.