This is really basic, but make sure you have your shoes sorted. They should be broken in and you should be confident you’ll be able to do all of your planned activities in them … and maybe some activities you haven’t planned. I like a pair I won’t be too ashamed to try to go into a nice-ish restaurant with, but most importantly I need to know I’ll be OK on my feet all day, I can run for a bus, train, or plane if I need to, and that the soles won’t be useless if I decide to, say, climb around on wet rocks.
My favorite travel accessory is a thin sports jacket with interior and exterior pockets and a hood that won’t blow off or obstruct my vision. It works year-round either solo or as a layer, and means I can often avoid carrying a bag — although I usually have a collapsible backpack folded up in a pocket in case I acquire a larger item or need to carry extra water (you always need something waterproof that zips shut for stashing your paper tickets, passport, electronics, etc., if necessary).
On my first trips, I brought a carry-on sized piece of luggage. Nowadays I carry only a backpack that will fit under the plane-seat no matter how long the trip, and I am frankly still overpacking. But there are some items I may not use yet never regret bringing, just in case: a hat with a chin-strap and a generous brim, a nice beanie for cold or bad hair days, sunglasses, lip balm, sticky notes and a couple pens, some pain medicine, something for seasickness, and an antacid.
You don’t need to plan too much in advance, but there are some big tourist attractions you won’t be able to fully experience unless you’ve secured tickets sometimes months in advance — so you do need to find out what those are to make sure you don’t end up heartbroken from wasted time, money and emotional energy, especially if you’re not sure you’ll go back. Related: some activities such as caving, helicopter tours, or even climbing a tower may be restricted by weather conditions, which may shift quickly. This means that if you have your heart set on any such activity, you will want to allow some room around it in your schedule in case you need to allow time for the conditions to be in your favor.
Finally, having a battery case on my phone has meant I usually get to charge my phone when I’m ready to, overnight if possible, and that I’m the reliable person in a group when we need to look up something or make a call after we’ve been trekking around for awhile.
On that note, don’t forget to download any music you’ll want on your trip, especially if you will be renting a car for any long, remote drives. On my first trip to Europe I needed to stay in a line overnight, and I realized I had only a single song actually downloaded on my phone, which I proceeded to listen to over and over again for hours while trying to keep myself awake. Fortunately my neighbors in line were good company.