Hi Deena!
I mostly use a notebook with pen and paper which is mostly in bagpack. I mostly write when up and about. Notes for mac which is a application just for typing in notes as the name says. When I don’t have an internet connection I use signal the messaging application which is free in the google playstore. I really like markdown too needless to say.
My tips for good experimental notebook would be first making sure you write down the date and when you wrote it maybe an idea you want to try and organizing them in good manner. I may use the resources above but It’s often difficult to find where I wrote my ideas especially in the notebook because it’s mixed up with bio projects as well. When I’m doing deep learning experiments i make a .txt file which I end up writing in markdown style 
where I record my trials for example here’s one of my entries:
Trial 3
In the next trial i want to a flip plus an flip_vert on ds_tfms part change the architecture to a deeper model resnet50 series with a learning rate slice of between 1e-3 to 1e-4. The accuracy was pumped upto > 95 that is, state-of-the-art right? I saved it as model 5. Now to serve it online and test it some more.
I hope that’s useful. I try and follow François advice too https://livebook.manning.com#!/book/deep-learning-with-python/chapter-9/point-4207-33-41-0