How do you plan for a game session when your PCs can go anywhere? This is a problem that I am facing in one of my games, and that a couple GMs for games I am in are facing. See, once you give the PCs the ability to transport themselves, you lose a lot of the "control" you can have over a game. Why? Because at any point the PCs can just choose to go somewhere else and do their own thing. So how do you prep for that? Well, today I have some tips I plan to employ.
Plans To Move Happen At The End Of The Session
This is the easiest one. It works in a very simple way. Basially, instead of ending your session with the completion of the last job/adventure, you end the session a few minutes later. What do you use those few minutes for? Why, to find out where the PCs are going next of course. Lock in the location, then end the session with the PCs preparing to set out again. This gives you the time between games to plan what is going to happen - either on the way or at the new location.
The problem with this is sometimes you don't have time, or something will happen to change plans, but in general this gives you the best idea of what is going on with the PCs so you can prepare accordingly.
Come up with some generic adventure hooks. The kind of things that can happen anywhere. When the PCs get to a location layer these hooks out and see which ones they go for. This will generally get the players involved in something, allowing you to flesh it out more into the world for subsequent sessions. The trick is to have it be big enough that you get that time, but small enough or generic enough you can use it anywhere.
For example, in my Star Wars game I could use some plots of general Imperial abuse of aliens to try and hook the PCs. So on a world they could see some Imperial troops shaking down someone at a market place. If the PCs get involved they have the initial interaction with the Troops to handle. This can take time and could lead to a combat. Then my job is to hit them with more of this plot before they leave. Maybe the troops came back after wards and arrested someone. Now the PCs need to get more involved. Around this time though, unless things are going very quick, we are likely near a session break or pause. This gives time for me to flesh things out even more and link stuff through the world.
Now, the downside to this is you need more prep and you never know what will work. Maybe my PCs walk right past that, so I also have someone selling fake Jedi mementos on the black market, and I have a kid looking for his parents who were lost during a raid from some bandits recently. The PCs could walk past all of these, or engage on any of them. If they walk past them...then we're kinda left with not a lot happening. If they engage on all, well you could be there for a while (not a bad thing.)
Mostly though, you'll end up not using some ideas. That is ok though, if theyre generic enough you can use them again later on.
The Home Base World
A lot of shows and books, even when the PCs can venture around, will have a central hub. This hub is a reset place of sorts. Jobs can take the PCs away from the hub, but they always end up back there. It has their home and where there business is. It also means that you have a location the PCs likely care about to use as a launch point.
The fun in this is the jobs offered can take the PCs places. This gives you some say. You offer the job, the PC bites, and you go from there. More prep on your part, sure, but you also have more say in where they go since it is unlikely they will go somewhere there isn't a reason to go.
The downside is it requires the PCs to set down roots somewhere. Some groups like this. Other groups like the wandering. Your mileage will vary depending on your group.
Improv is Key
The big thing for all of these is to learn to think on your feet. Take a minute if you need it, but get used to using with the bare essentials and adding fluff as you go through. Let the PCs drive the action when you can, because they'll often cause their own problems to make their own adventures.
These three methods are tips to help add some structure, but it is by no means exhaustive. Best of luck to you on it.