As soon as you gain access to Beast Shape II, which should be at 6th level for the vanilla druid, you turn into creatures that have the Grab ability.
Grab (Ex):
If a creature with this special attack hits with the indicated attack (usually a claw or bite attack), it deals normal damage and attempts to start a grapple as a free action without provoking an attack of opportunity. Unless otherwise noted, grab can only be used against targets of a size equal to or smaller than the creature with this ability. (...)
Creatures with the grab special attack receive a +4 bonus on combat maneuver checks made to start and maintain a grapple.
So, unless you really feel you need the extra +2 from Improved Grapple, you do not need to spend two feats simply to be an effective grappler.
Greater Grapple, however, is an upgrade to your grapple checks, but again, your real benefit from the feat is the ability to maintain a grapple as a move action instead of a standard action.
This will cost you three feats, from which you will only benefit partially from one and recieve the normal benefits of the other. Again, on a class that does not gain any bonus feats, so investing on grapple feats are not very useful for druids.
