Every client has their own list. New to-do takes a title and optional notes, and the list can be reordered by hand — the top of it is what you have decided to do next, not what you happened to add last.
The list of everything
There is also a single view of every open to-do across all clients, grouped by who they belong to. That is the one to open when you are deciding what today looks like rather than working on one account.
Ticking things off
A completed to-do stays visible on the client's own list for a week, so you can see what you did rather than watching work disappear. The list of everything shows only what is left.
Timing a to-do
A to-do can be handed straight to the timer. That starts the clock against the right client with the to-do's title already filled in as the description, which means the hour you log says what it was for without you writing it twice.
This is the small join that makes the record worth keeping: the thing you planned to do becomes the description on the hour you billed for it.