I'm not an expert in this space so I will start with the disclaimer that you should immediately stop any of these exercises if they bring you more pain. A bit of of soreness is to be expected though as you begin if you've been out of practice for a while.

Here's a short cardio workout option


Here are some ab exercises you could try


This is a low impact chair workout. I'm not sure if any of these moves would flare up back pain, but it may be worth a shot


Also, if you type 'exercises safe for back pain' into the Youtube search there are a bunch of options for stretches and yoga exercises that are meant to assist with the pain itself and strengthen your back. Working some of those options in seems like a good idea.

Start slow and work your way up. Any changes you make are valuable!