What to do when there's nothing to do?
When the heat is on and you are defending and/or attacking, there is never enough time to do all you want. On the other hand, in quiet positions where nothing seems to be happening, it looks like there is little to do and lots of time to do it.
In such positions it is important to make moves that advance the game in the right direction. To do so you need to have an idea of the future shape of the game, so that you can push it in that direction. Every move should contribute to build that shape in a favourable way. It may not seem to matter whether the rook goes to d1 or e1 but the seemingly unimportant choice can have a pivotal effect later in the game.
So you need to play out long sequences of moves to see where the game is headed.
When in doubt, think of centre control, piece mobility, defence of your weaknesses and pressure on your opponent's. Another factor to remember is that in the endgame the position of a pawn on g6 rather than g5 can be the difference between winning and losing.