You need to learn to recognize the signs of your brain starting to struggle. Needing to rereading a line of text is a sign your brain is struggling. Having to squint or look again to focus and recognize an image is also. You need to find these signs yourself because they are individual to you. Just because you can push through a struggle does not mean you are OK with that task.

Hawaii should be OK if you avoid crowds. the flight may be your biggest struggle. Rest up before the flight and plan on needing to rest after the flight.

If you feel the need to do web design work, start with pencil and paper sketch ups and diagrams. This will get the cognitive work started without the screen stress. Then, when you sit at the computer, do it in stages/steps with breaks at the first sign of mental/cognitive fatigue.

I needed to use this stages protocol for years. I was like a butterfly flitting from task to task.

My best to you.