Hello,
Basicly You need to know that to make good looking natural hoovering obove the ground is not so simple task and it can be a little to early for You 
using a curve seems to be good idea, but the downvote for it is that it will float always in the same path. Using behaviour tree for something such simple as hoovering powerup is a bit overkill IMHO.
The way it shoul be truly done is to use noise. Noise is a algorithm that gives You numbers that are random but each subsequent is connected with previous and similiar to it so it is good to animate position. Google “Perlin Noise”.