The one last thing that is left to do is to breathe life into your sandbox. You do this by requiring players to play through the lives of their characters between the adventuring - leveling up, acquiring knowledge, spending wealth, healing, and keeping track of the time that they do. Gary Gygax writes in his DMG about the importance of keeping track of days between the dungeon expeditions for the purposes of keeping track of the party members as they go about their separate ways between the adventures and have to get together for the next adventure.
The way you set your sandbox in motion is by creating a set of random events tables to cover your world. Usually yearly events at national level, seasonal events at regional, and monthly events at the local level of the city, town or village, where the players are based. In addition, there should be weekly events in the lives of your players. This, and the wilderness map key, is your real and only chance to immerse your players in your world and show them what is in it. Does your world have volcanoes or purple mountains? Dark forests or elven cities with landscaped wilderness? Your area maps should show these details. Events likewise, should show off your unique setting. Pick up Gary Gygax's Oriental Adventure supplement, that he published in 1985. In it he invented the use of random events to make the players' setting be reminiscent of the classical Chinese literature that he bases the setting on. He breaks the major yearly events into three tables - Sociopolitical events, natural events, and special events. There are weddings, princely births, comets, famines, peasant uprisings ands visiting armies. This is your chance to express your unique and original setting in terms of major events that happen in it.
The second book that you may find helpful in The Insidiae, which I reviewed previously, the Vol. 5 of the Gygaxian Fantasy Worlds series. Essentially, that book is the version of the Oriental Adventure created for the more storyline centered conventions of the AD&D Second Edition game rules. It features NPC centered adventure design - you create NPC's, give them a role in the story of your players, and the adventure hooks come out of their motivation. I don't like it for many reasons, primarily because it was written by someone who is neither a literary scholar, nor a genuine gamer, so the ideas seem a bit abstract and generic. Also I don't like the casualness in which powerful magical items, demons, and other fantasy elements are treated. Everything is mundane and is taken for granted. Where the Insidiae book works wonders for me, is in setting the timeline for each area in your campaign setting. You get to flesh out the underlying issues and the direction of the historic development for each individual region. When I tested it on my own Midlands campaign setting, it coalesced beautifully with my own ideas about the place and formed a rich backstory for the place, a neighboring Barony to the east of Barony, where players are based. I could also use the Backstory as the inspiration for the random major events that will be scheduled on the campaign calendar, that will include fairs, celebrations, religious festivals, that happen regularly in that fantasy world.
Finally, with the fantasy world in motion, we need to create events that affect your players to distract them from their daily business of life. Keep in mind that your thieves and magic users will be affiliated with guilds, your players will be somebody's warriors or men at arms or knight errants, if they have the money to afford it, etc etc, every player will have his or her routine and some kind of minimal affiliation with the outside world. Into this routine, you throw random events, which cover events at their guilds or places of business, encounters with friends, enemies, patrons, family members - NPC's of significance. Players own backstories are a great source for random events for the player characters - mostly people from their claimed (by the player) past coming to visit them and sometimes player characters' past coming back to haunt them. That is why I say, anything that is said in the campaign is CANON, whatever players want to claim for their charters is fine - exiled princes, demons, sons of King Arthur, it's all good! No need to roll for 3d6 starting gold if your dad is a rich vampire! Of course, a good DM can twist all these fairy tales into deadly and terrifying adventures for players! Another thing I like doing, making players roll 3d6 or 4d6 pick the highest against each ability. Players can roll up a smany sets of stats as they like until they get the character that they truly like! DM saves all of the rolled sets of stats. These all will come into game as NPC's from the player character's past trying to kill them.
And there you have it!