A week or two ago I was poring over the forums at dragonsfoot and came across a thread about characters wishing to build or buy a tavern in-game. Most people seemed pretty neutral on the idea, with one or two wondering why players in a D&D game would want to concern themselves with a mundane enterprise like running a tavern, and a few looking at the up-side potential.
This was of interest to me at least in part because my players back in the day were wild for in-game business ventures in general, and their characters owned their share of taverns. At least in the case of my players, I can say with confidence that they were not at all interested in the minutiae of running the business. They were not keeping track of inventory, worrying about current prices of ale and wine, tallying up daily receipts of silver and copper, or fretting over maintenance of the premises. Why, then, would players want to own a tavern or other business in the game world, and why would the DM let them?
It's a little bit about fame and prestige in the game world. Buying or building an inn or tavern represents the point at which a party of characters graduates from grubby murderhobos to genuine members of the community. It's a step on the ladder that's within reach of low-level characters, long before the giddy heights of name level and titles of nobility.
Even more than that, it represents the players taking on a personal stake in the game world, or at least their chosen home town or village. Expressing a desire to open up a tavern is expressing a desire to be a part of your fictional society. Town isn't just the safe zone where they're allowed to rest without wandering monster checks and erase gold and add equipment to their character sheets; it's become a place in the shared story that they care about. And they'll care about it even more once they own a piece of it: Orcs raiding in the countryside and bandits preying on traffic on the high road aren't just opportunities for mayhem and loot; they're calls to protect the players' (fictional) financial and (real) emotional investments in the game.
An inn or tavern is a place characters can call home and store their loot, but it's also a place that attracts interesting people. There's no better place to hear weary travelers talk of faraway lands. Bards and minstrels will stop and regale the locals with their songs and stories, too. That's true in any watering hole the characters might visit, of course, but when you own the joint, you can have your serving staff keep an ear out for things even while you're not there, and you can do everything in your power to make the place more attractive to the best performers. The better the inn or tavern, the better the quality of the rumors you can gather there.
Speaking of NPCs, a position at an inn or tavern is a fun way for the PCs to reward less fortunate NPCs that the players would like to keep around. The beggar who provided them a vital clue, a peasant rescued from a goblin lair, or a former retainer who has given up the dangerous life of adventurer's assistant might all find new life as tapsters, handymen, grooms, or barflies in the PCs' establishment.
Lastly, an inn or tavern can be a source of income, but that should probably be much less than characters can make by adventuring. Most of the proceeds of the business will go toward inventory, maintenance, and wages for the folks who run the place in the PCs' absence. 3d6 gp (or whatever your base coin is) profit per month sounds about right to me. Perhaps adjust the amount depending on how much traffic the location gets - less for a backwater hamlet, more for a major trade route. The PCs may pocket the profits or reinvest them in improvements. If they manage to make their business truly exceptional in some way, extra d6s can be awarded. Dealing with losses and shortfalls is usually no fun, unless it's used as an adventure hook: Trolls have taken over the bridge north of town, or the baron just across the border has imposed a stiff tariff on goods entering the domain, and traffic has dried up. Time to strap on the armor and weapons, dust off the spell books, and show them who's in charge here!
Inns and tavers, for the reasons discussed above and more, are the most obvious choices for PC investment, but others are possible too, such as pawnbrokers, blacksmith shops, or even trading or shipping firms.
Done right, a PC business provides many interesting ways for players to interact with the game world and its inhabitants. Just leave the quill pens and accounting ledgers in the background where they belong and focus on the possibilities for adventure and intrigue that open up when players are willing to invest their characters' coins and their imaginations in having their very own piece of your world.