The following links explain how to run a sandbox game with a plot and with a minimum of effort, just in case you need them (mostly, because these are links to forum discussions where people more experienced than me explain how they run sandbox games).
Now, first of all, remember that running a sandbox is anything but
hard. If 9-years-old kids can do it, and they can, so can you. An important part of the tricks is about minimising the amount of work you need to do! That's why the first three are links to what the authors had called "Lazy Game Mastering", lazy here is "not doing work you don't want to do".
And here are the links explaining some tricks of the sandboxing trade
Also, check the GMing advice (and only that-it's independent of whether you use the same system, or any system at all) in those three systems. While not the best explanation on how to run a sandbox game, they're the best free ones.http://rpg.drivethrustuff.com/product/115280/Turbo-Dicehttp://rpg.drivethrustuff.com/product/124052/SciFi-Dice
In these two, pay attention to the requirements to the GMs and the players... the latter are worth posting in your Opening post as "expected from players".http://rpg.drivethrustuff.com/product/70124/Atomic-Highway--Post-Apocalyptic-Roleplaying
In this one, stuff for organising the session.http://rpg.drivethrustuff.com/product/86467/Stars-Without-Number-Free-Edition
This one is very old-style and might not be altogether helpful, but skip to the GMing section and it should have a nice system with tags you can adapt for your own notes.
Also remember the GMing advice from "Beyond the Wall" (young adults fantasy).
1. Keep things moving
2. Make it personal
3. Avoid illusionism
Illusionism is a term used to describe a particular problem with seat-of-the-pants GMing in roleplaying games. Illusionism happens when a gamemaster makes sure that every choice the characters make is the right choice (or vice versa). For instance, the characters enter a small cave and come to a junction. One way leads to the end of their quest, the other to something else. The GM waits for the characters to say which way they are going, and then immediately places the end of the quest
in that direction.
This is certainly not necessarily the worst thing in the world, but it can make some players feel that their choices do not matter at all, and they would be right.
If the evil wizard will be down the right hand path if I choose it, but down the left if I choose that way instead, then why did I choose at all? Why didn’t the gamemas-ter just make a single hallway taking me straight to the destination he had in mind?
You can avoid Illusionism fairly easily, even while running things on the fly. If there are two ways to go, simply decide which way leads to the evil wizard, even if you only make that decision five minutes -- or five seconds -- beforehand.
Let the players fail, because those failures will make their successes all the sweeter. It’s okay if they stumble off the track, or reach a wrong conclusion. And if they succeed a little too quickly and easily, let them! Sometimes they deserve it.
(Let me make this clear: I'm very much one of those players. If I'm playing a sandbox game, it's about the freedom to do anything and get both the best and the worst consequences of what I did. But they're the consequences of what I did, not of someone's idea of what I'd enjoy, nor of anyone's ideas for story pacing (which are also easily disproven by many successful movies and books - TH).
4. Keep the tone suitable for the genre