What is the best way to spend your time when its your opponents move?

So I really need some suggestions because I'm just twiddling my thumbs while my opponent is thinking.

With no immediate tactics in the position, try to improve your worst placed piece.

'When my opponent's clock is going I discuss general considerations in an internal dialogue with myself.
When my own clock is going I analyse concrete variations.' - Botvinnik

I forget where, but I once saw the suggestion to spend the time, looking for mistakes that one will want to be sure to avoid.

Try to come up with more general plans rather than specific moves if it is your opponent's turn. Since you don't know what your opponent is going to play yet (unless they have an obvious best move and they're taking a long time for some reason), calculating specific lines is likely going to be a waste of energy if they don't play what you expect. But also don't worry too much about thinking really hard during your opponent's turn, because you only have a limited amount of mental energy, and if you're constantly thinking at 100%, you'll burn out. Your opponent's time can also be used as a time to rest and recover mental energy for calculation during your turn.

(And I don't think the advice in #2 is very useful. I had some other words, but my post got deleted, so I'll leave it at that.)

If you’re twiddling your thumbs, the most practical thing you can do is focus on preventing their plan.

Ponder on their time searching for the problems they could be brewing up.

If you identify those threats ahead of time, you’ll be able to reply effectively, which indirectly will also help you save clock time.

Strolling around the tournament hall.

Playing chess creates a lot of stress and adrenaline builds up. Some people like to walk around while not on move and that helps dissipate the tension. Of course, you can still think about the game and it can help to take a step back and look at the big picture.
If you are at the board, it depends on the type of position. If it needs concrete calculation, it's certainly helpful to use your opponent's time as well as your own. If it's quieter, then you can think of general considerations, what blunders to avoid or induce and what you're going to eat afterwards.
In the 1971 Candidates final match between Fischer and Petrosian, the lights went out. The game was paused but Fischer continued to sit at the board and Petrosian complained to say he was still analyzing. Fischer said, no problem, and allowed the clock to be restarted in the dark.

@Hagredion said ^

Strolling around the tournament hall.

XD honestly thats my favorite pass time ...