+1The goal of sandbox play imo is player freedom and the feeling that they live in a changing open world.
I achieve this in my campaigns with the following:
1. Ask players what they intend to do next session. This should help you a bit with more focused prepping and therefore is crucial. You can devote the little prep time to fleshing out that particular ruin where they might want to go or the route there.
2. Have some sort of "timeline what will happen if PCs do nothing" or "goals for NPCs and factions and how they pursue it" -system. I would recommend checking out various PbtA systems (Dungeon World etc) on how they handle what they call "Fronts", also Forged in the Dark games handle factions with clocks etc. There are many ways to achieve this.
No matter what kind of system you use the idea is that the world does not revolve around the PC's. Have NPCs have goals and what will happen in what timeframe if PCs do nothing. Note that to get them interested you need to inform them via rumours and news etc. about what is happening in the world during every session.
You want to have Coven of Hags in the nearby bog as a factor in your campaign. You want them to try to invade/curse a nearby village
Perhaps you give PC's legends about them and treasures they have but they are not that interested. So the hags go on with their devious plan.
1. Rumours about sickness in the nearby village, dead fish turning up in river
2. Same village has some people dying of this weird new sickness, there seems to be no cure and local druids are worried about plants dying as well.
3. Some of the dead villagers start coming back as undead!
4. Village is on the brink of disaster, they send out call for help with a promise of reward but no one dares to go there because of rumours of a powerful curse
5. Village is destroyed by the hags. Populaion completely turned undead.
What will be the Hags next move? You decide.
At any point the PCs might intervine. When to advance from 1 to 2 to 3? You determine the rate and of course PC actions affect it as well. Maybe their actions slow hags plans or completely stop them. However, if they do nothing the village is destroyed and world is changed. Overall I advance my campaign plots at the rate of one step every 1-3 sessions, you want to give PCs time to act especially if you have few plotlines in the background.
Now have a couple of plots/factions like this in the background and tie that in with legends and locations and artifacts. Perhaps a particulas artifact lifts hag curse? Perhaps a remote monastery has plants that cure illness? This gives players a sense that there is a living breathing world around them and they have the power to affect it.
Final note: good sandbox campaign requiers player buy in more than other ways of playing. If the PCs have goals and motivations and they actively pursue them it runs beautifully. If they expect GM to tell them what to do it pretty much loses its purpose. In FBL Dark Secrets can easily be tied into sandbox plots btw, as can strongholds...