Motivation and discipline. I have found adding it to a list helps the discipline part for me since I'm OCD and have to finish all the lists! :)

Thanks for the recommendation, but is it possible to do something for the back without a bar to do the pullups? I dont have a bar and I cant install one on the apartment I rent. Thanks!

There are pull-up bars that use only friction and leverage, that don't require any drilling to install. Here's an example: https://www.amazon.com/Iron-Gym-Total-Upper-Workout/dp/B001E...

They're a little low and only support <250lbs, but otherwise they are perfectly fine for the average home workout.

If your doors are sturdy, make a knot in a towel, hang it over the door and close it. Use that to do pullups. I used to do this a lot during traveling.

You can also do inverted rows under a table by holding on to an edge (not comfortable, but works).

You could invest into a dumbbell and/or resistance bands and do row variations. Or a TRX for inverted rows.

Or go to a park or anywhere else where you can hang onto to do your pullups.

Abs: sit ups. Too easy? Hold something heavy against your chest. Too easy? Hold something heavier.

Fault! This is adding weight. Better luck next time. :)

Hanging leg raises are what you're looking for. Start with bent leg, then straight leg, then toes to bar etc.

Pilates (wothout reformer) offers harder ab exercises without external weights