Adding onto Pam’s suggestion for journaling. It is difficult to remember what you saw when several days have passed. I bring a notebook, but I’m usually only good at writing in it for the first few days. Then it’s just what I ate and a running note of costs, for planning future trips.
I’ve found that it helps me to take a photo of the info about where I am. For instance, I will take a photo of the entry sign (or gelato shop sign!) or art museum piece sign. Then later, I can add a few notes on the photo I want to keep on iCloud and delete the info sign photos.
Another photo helper is to take a photo of your room key when you’re in each hotel room. I move locations quite often, and when some of the keys are kept behind the front desk lobby, I have to think to myself occasionally, “What IS the room number?” : )