Roll up knickers. Put into a ziplock bag. Squish all the air out and close. Saves space. Ditto socks and bras.
Pack mix and match outfits, plus something fabulous you'll wear once.
Pack lots of shoes - you never know!
Pack flipflops or sliders for the hotel room - I never walk in bare feet in hotels
pukeface.
Pack a A4 wallet or envelope type thing to keep all your paper souvenirs in (or is this just me? I always pick up paper maps, pricelists of things I've done -horserides etc, entrance tickets to places). Only if you don't Kondo 
I research the fuck out of places, look at prices, options, discounts, times, off the beaten track type stuff, Google Street view the hotel and surroundings, look out for restaurants, look at their reviews etc. We're hoping to go to Bilbao soon and I'm having enormous fun looking at what to do! We won't do half of it, but if we leave the Guggenheim and fancy a cheap lunch we can go to X 10minutes walk, if we're Hank Marvin and and have a good budget we'll go to Y place 15 minutes away. That kind of thing. Or of we're walking from A-B, we could go down C Street cos that's where they filmed James Bond.
That type of thing.
At least if we don't do some things, we'll save them for the next trip, if we liked the place.
And take a book/sudoku for downtimes when you want to put your phone/camera down and just soak up the atmos.