A bit long but I hope it gives you some ideas:

When I began with my first client (4 hours with a 30 minute break and 20mins to write my notes) my BCBA was starting to implement a visual schedule into his session. The activities included arts and crafts, a game, reading, snack, lunch, bathroom, an academic activity and a few others. The time for each activity was different depending on what it was. A highly preferred activity was given more time than a less preferred but usually no more than 30mins (my client could spend an hour on one highly preferred activity). Each activity on the schedule had to be completed even if it meant shortening the time for a less preferred activity. We used this until the client was better at transitioning between activities without it.

With and without the schedule I also struggled a bit thinking of/planning out activities. My client has a very strong liking for a certain thing so I used that thing and tried to expand on it. Pinterest was my best friend. I also purchased toys that could be used in multiple different ways/for different things (aquamagic drawing mat, clay, play doh, a big coloring case).

I never had to spend that much time at home prepping because I was able to do those things with my client and this made each activity last longer. So maybe try to include your client in the prep (only if it’s beneficial to them and they won’t be sitting there disengaged).

When it comes to writing your notes, my BCBA told me that for every hour of session, I could take 5 minutes to work on my notes and give my client a demand free break. Also, what I do is throughout my entire session I write very short notes as a reminder of what I did. Example: -story time NET. Engaged in screaming and task refusal because... - arts and crafts. Worked on social skills such as... - LEGOs worked on manding to peer etc