Every time I come back from a trip, I am bursting with travel ideas and advice. This entry is a little all over the place, but take what pertains to your flavor of traveling!
Once you've seen a view, you've seen all of them
I think it's worth it to see one high panoramic view of a city but don't keep paying for it over and over again. Small tip: you can get gorgeous views from rooftop bars while enjoying a drink which is often cheaper and faster than paying a tourist ticket and standing in like for a gazillion hours
When in markets, go deeper in because you'll find better deals
The goods on the outskirts of a market are often the most expensive because people are too scared / lazy to go explore internally. Depending on the labyrinth of a market you're entering, I'd recommend finding a particular good you're looking for as you go deeper in to haggle for.
Packing and souvenirs
I'm a fan of packing very light (took only a backpack and a duffel bag for a 2.5 week trip through Morocco, Spain, and Portugal). The thing I really think about here is "What is actually something I will value taking back?" Magnets are great, but how many magnets can you really collect? I always research souvenirs to buy (or spend the first day walking around to see what's popular) before actually purchasing anything. I'm a huge fan of postcards (which I send to friends when I get back), and tend to find interpretations of the landscape (aka art prints) rather than just photographs (because you're likely already taking photographs that you can go home and develop).
Plan your daily route home otherwise you'll suffer
Just as you planned your trip from a higher level of creating a loop to get back (assuming you booked a roundtrip in and out of one city), plan your days according to geography. Plot your interest points on a Google Map so you know at which point you can head home.
Keep in mind to see things during the day and at night
This past trip (Spain and France), I got quite sick. I ended up waking up late, explored for a few hours in the morning, napped in the afternoon, and then woke up again and went out. This was the best travel hack I think I've realized to date. I was able to keep up my energy and really thought about what I wanted to see in the daytime vs. the nightime. Of course there are points of interest you may want to see at both times (like the Eiffel Tower), but just means you have to be more conscious of those routes.
I try to research / ask friends what were the most popular foods they tried in a particular place. Making reservations is great (depending on your budget and what restaurants you're looking at), but sometimes it's worth just having those foods in the back of your mind as you're wandering around and get hungry.
I have invested in a jacket with zippers for pockets when traveling between cities. This allows you to have your phone, wallet, and important documents on your body. I would also recommend not having important documents on the first outer zip of your backpack / duffel bag, etc. While this provides easy access for the traveler, it makes it very easy for a pickpocket. In high-density tourist areas, I will also wear my cross-body purse underneath my jacket so that if someone tries to pull it, they would literally have to rip off my jacket to get the purse.
Keep your space
As a solo female traveler, I recommend always leaving a stair step space between you and people on the escalator. Silly and small, but hey might safety first.
Don't look at your phone plzzz. You are in a different part of the world and all you can do is stare at a piece of glass and look into people's lives that aren't around you? Use meal times as a moment to destress, think about what you want to do next, people watch, and truly enjoy your food. I personally think eating alone is something everyone should do regularly in their lives to get some perspective on how to spend time by themselves (and value time with others).
Having exact change
This might be a personal travel pet peeve of mine, but in countries where credit cards are used less frequently and cash is still the main form of payment, have exact change. Holding up the line, and then handing over a ridiculously large bill or coin just frustrates everyone. Be conscious of the cash that you are carrying and if / when to get change according to the purchases you're making.
Turning keys and locks
As travel-savvy as i try to be, I really struggle with keys and locks. I don't lose them but I have such a hard time turning locks. This sounds ridiculous but it was a key revelation to me that when locking the door, you turn the key toward the lock or the seam of the door (usually clockwise) and when unlocking the door, you turn the key away from the lock or the seam of the door (usually counterclockwise).