I have no sand arena but work in the pasture or while we are going for a walk. Concerning the grass and finding it more interesting: When we go for a walk, I have a rule that the horses cannot graze unless I say it's okay. However, when we are in an enclosed area (in the pasture) and the horses are at liberty, I see grazing as a most valuable feedback: If the grass that they have all day is more interesting than me, then there is a lot of potential for me to become more interesting.
You will find some ideas for how to start in the groundwork exercise subforum.