Back when I worked in a commercial gym, I'd often catch my colleagues sitting down for half an hour planning their next client's workout. As a general rule, taking the time to think about what your client needs and how you can structure that is a good thing. But when there's nothing special about your client (no health issues that require specific programming, no injuries, the client's goals revolve around general fitness and looking better naked), do you really need to spend that much time on each client's next workout?
For the average client with average goals and no special circumstances, here's a simple template you can follow for each workout:
5. Abs or Loaded Carry
Now, let's pick a few different exercises you can do for each of those 5 categories of exercise:
1. Legs: back squats, goblet squats, leg press, step ups
2. Push: bench press, overhead press, push ups, dips
3. Hinge: deadlift, RDL, hyperextension, bridge
4. Pull: pull ups, lat pull downs, rows, upright row
5. Abs/Loaded Carry: sit ups, planks, side bridge, farmer's walk
So, that's 5 movement types, and 4 exercises per movement type. That's hardly an exhaustive list, mind you. I could quite easily list off at least a couple different variations of each exercise on that list, still without really thinking about it. And there are plenty more movement types you could consider. Very often, when trainers early on in their careers spend so much time thinking about the next workout when there's nothing that they need to specifically worry about other than keeping things balanced, it's because they believe that they need to use a lot of different exercises each session in order to keep the clients coming. Depending on the clients, this may be true, although that isn't an absolute statement. Nonetheless, with 5 movement types and 4 exercises per movement type, you've got literally hundreds of different workouts that you could potentially give your clients.
That's not taking into account coaching, the many different options for progression (increasing weight, increasing reps, increasing sets, increasing ROM, decreasing rest, etc), "advanced" techniques, exercise arrangement (3x10 sets across, supersets, circuits, targeting reps, targeting time, etc) and the many other variables that you can play with.
It's my opinion that a fair amount of forethought should be given early on, but if you're spending a lot of time planning every single session when there isn't a clear need for it, then perhaps you should be focusing that attention on more pressing matters. If you understand exercise programming and have a well-reasoned system sorted out, then (for the most part) less time should be spent thinking about what to do next, and more time should be focused on how to use what's coming next.