I have been working on this same kind of project for Oblivion for a year or two, though lately it has not progressed too much because of Skyrim's release.

Here's what I've been adding to the taverns to give you ideas.
More NPCs
Here's a list of patrons I've added to the taverns (Put in spoiler tags so the post is not too clunky

Many of those are set to dance to the bards' songs.
- Assassin
- Barbarian
- Bard
- Hunter
- Warrior
- Bounty Hunter (If player has a bounty of 200 or higher and a bounty hunter is present in the tavern, he will confront him about his crimes and ask to turn him to the guards)
- Champion
- Commoner
- Miner
- Thief
- Imperial Battlemage
- Imperial Legionary
- Imperial Captain
- City Guard
- Hold Retainer (A retainer for each hold. Traveling around Skyrim as ambassadors)
- Knight
- Mage
- Traveling Merchant (Can sell wares)
- Nobleman/Woman
- Pilgrim
- Priest (Of each Divine)
- Rogue
- Traveler
- Acrobat
- Monk
- Healer
- Vampire Hunter (Equipped with vampire hunting equipment such as flaming weapons. If player is a vampire, the vampire hunter will confront him and provoke him in a duel)
- Werewolf Hunter (If player is a transformed werewolf, he will immediately attack)
- Freelance Fences (Can purchase/sell stolen goods. They are rare)
- Agent
- Crusader
- Battlemage
- Dark Brotherhood Assassin
- Nightblade
- Savant
- Scout (Can reveal interesting locations on the map for a fee)
- Sorcerer
- Spellsword
- Witchhunter
What I did in my Oblivion mod was having around 75% of the city inns populated with locals (Peasants, Commoners, Noblemen) and the other 25% with Adventurers (Archers, Agents, Mages, etc.) and other city individuals (Guards, Local priests) while the wilderness inns have only adventurers, farmers, hunters and patrolling soldiers.
Regarding races, I have 50% of the NPCs as the local race (So Imperials in Oblivion), 25% of the race closest to the bordering province (Redguards in Anvil, for example) and the other 25% as the rest of the races. So, for instance, having 50% of Nords in Markath, 25% of Bretons and 25% of the rest.
And I think it's now possible to easily create groups of NPCs that can only spawn together, if I judge by how some group of NPCs spawn at random encounters on the road. That can make it easy to have a group of adventuring buddies spawn only together at tavern instead of having only patrons coming by themselves. (Or couples, groups of commoner friends and the likes)
Oh and I was going to forget, some inn employees. Such as the cook, the bouncer(s), the waitresses that greet you when you take a seat (Like in Whiterun). The bartender could also greet you if you sit at the counter.
Unique Dialogue
The NPCs can talk about your player's actions that they overheard. And since it's mouth to ears, these rumors can be wrongly described. Such as saying that the Dragonborn can shoots bolts of lightning of his eyes and fireballs from his buttocks.

Minstrels can be tipped. And since minstrels and bards stay in taverns, they can know inform the player about the latest rumors for a fee.
Regarding Horses
A leveled creature marker can have a random chance of spawning horses at the stables. That way, a inn can have one, two, more or no horses at all from time to time; Creating the illusion of come and go from mounted travelers.
Atmosphere
Some unclean taverns can have a rat or two in the cellar. Since rats are attracted to those establishments, having cats in the cellar or on the main floor adds a bit more atmosphere.
It's a bit different in Skyrim since there's one tavern by city, most of the time, but I'd suggest having different social class patrons for some establishments. For example, a high concentration of poor Dark Elves and other non-Human races at the New Gnisis Tavern in Windhelm or more nobles, knights and crusaders at the main tavern.
I'll be following this. It's in my plans to mod in some tavern mini games, such as drinking contests. (I have scripted one for my Oblivion tavern one that works wonderfully.) It'll go perfectly with your project. And I'll also work on Sounds of Skyrim, adding new tavern sounds to enhance the atmosphere.
Good planning!
