Four figures, once a week, ten minutes. Bookings taken, revenue, no-shows and how full next week looks.

Bookings taken this week
Not appointments delivered, bookings made. It is a leading number, so it tells you about next month rather than last month, which is the only kind of number you can still do something about.
Revenue
Against the same week last month and, if you have the history, the same week last year. Seasonal businesses that compare against last week alone frighten themselves every January.
No-shows
Count them. Most businesses feel no-shows rather than measuring them. The feeling is usually worse than the number. If the number really is bad, deposits are the answer.

How full next week looks
Gaps you can see on Monday are gaps you can fill. Gaps you notice on Friday are lost.

What to do with them
Write them down somewhere. Four numbers in a notebook every Monday for a year is a better management report than any dashboard you never look at.
Common questions
Is once a week often enough?
Yes, for nearly every small business. Daily is noise.
Which single number matters most?
Bookings taken, because it is the one that tells you what is coming.
What if the numbers are bad?
Look at the gaps first. Filling a quiet week from your existing customers is quicker than anything else available to you.