There are two types of adventures; the planned, and the spontaneous.
Planned adventures include meticulous research, months of anticipation, organization, and when it comes time to go, you finally live out the trip like a dream you've had a hundred times before. The spontaneous on the other hand is a creature of opportunity—other plans fall through, there's an unexpected break at work, or you decide you need a brief escape.
These impromptu trips are a specialty of mine. My first was a decision to leave for a week-long trip with a friend to the Rockies in Colorado 9 hours after the idea came up. Since then I've gotten phone calls which had me in Shanghai a few days later, others have had me go from California to Norway to Saudi Arabia within a week. No matter what the trip is, these travels bring about a different and often more genuine sense of adventure.
Disclaimer: There is more room for error, for things to go wrong, but there is also space to say yes to more, explore authentically, and grow in your ability to travel. Some of my favorite travels and craziest stories have been the result of unknowns coming from last minute adventures. That being said, I have a few pieces of advice to make the most of your last minute plans!
Leave at the soonest possible time, then plan the trip backwards
Time spent trying to plan on the front end is often time lost on the adventure you're about to take. So book your departing flight and your return. From there, start at the end. Where do you need to be the night before to catch your return flight? Once you've got this peace of mind, planning the rest of the trip is a lot easier.
Only pick one "for sure"
The pitfall of quick trips is feeling like you're missing out. You will have lots of options, and the temptation is to try to do them all. My best piece of advice here is to find one activity you really want to do, designate time for that, and let the rest happen. Stay open to changing plans, know you will not do everything and this will be reason enough to come back.
Think about staying in a few different hotels
A lot of times people will stay in one hotel the entire duration of their trip. This seems easiest, but often choosing 2-3 different hotels ends up giving you a much better experience. You'll be able to see different parts of the place you're visiting and often be able to get discounted prices on certain nights.
Enter your dates on third-party hotel booking sites and then check the costs if you were to check in/out the day before/after your stay. You'll often see prices drop up to 25-30% by doing this.
Ask your flight attendants
Flight attendants are expert travelers who know how to take on a city in a day or two. They are often traveling last minute to new places and are used to exploring them without much research. They'll usually be able to get you off on the right foot.
Don't fall for any tricks
99% of the time locals are extremely helpful. However, one thing which can be a little rough about last minute travel is how sometimes people will look to cash-in on unsuspecting travelers. A simple internet search such as "*country you will be visiting* traveler scam" will give you things to look out for. Be aware of false "import taxes" on your bags, corrupt taxi drivers, and having to pay an extremely high bill for a casual meal or drink. Again, these are 1% situations. Just remember there will always be someone around you to help.
The most important one —Talk to a local
Chatting up a local is not just to find the best thing to do, but to have the best experience possible by connecting with another person. Locals will be excited to share about their home, but make sure not to treat them like a tour guide. I often connect with other photographers by asking to shoot together and then find out the hidden gems you could never find on a travel guide. You'll also find the best food this way.
Most people ask, "What's the best restaurant around here" and the answer will often be the fanciest place in town. People tend to think of where they would go on a special occasion. The question you should ask is, "If you'd been gone for a year, where's the first place you'd go to eat?" When answering this question, locals won't tell you about the stereotypical nicest place; they'll let you know about their go to's, the little kitchen you would never find, a tea shop in a back alley, or maybe even a dish their mom cooks and an invitation to come over.
This tactic can be used in person and over social media to find not only the best food but also the best hike, music, and activities. Again, this is a friendlier way to travel so make sure to treat those willing to share with you as a friend. Invite them along, ask questions, and share a bit about why you came. These types of interactions often create my favorite and longest lasting relationships all over the world.