I know there are a few dozen things you can do each day, but I'm just gonna post the ones I know of that are useful.

• The Dream World can be accessed every 20 hours.
• 12 Casteliacones can be purchased in Castelia City (and sold for a 1200% profit on Route 5).
• All Join Avenue shops restock.
• A guy in the lowest cave of Twist Mountain (accessed easily from Icirrus City and turning downwards at the first fork, except during winter, when you have to go the long way) gives out a free fossil.
• A guy at the aforementioned fork offers part of the TruQ quiz. (After finishing, you get a medal.)
• A breeder on Route 5 sells five of a random EV-reducing berries.
• By using the Dowsing Machine in Undella Town, many treasures (Heart Scales and Pearls) can be found along the shore.
• The Royal Unova sails.
• Colress is up for a battle at the Plasma Frigate next to the P2 Laboratory.
• Shops sell goods in Black City/White Forest.
• During the spring, Cynthia is up for a battle in Undella Town.
• The Ferris wheel offers a battle in Nimbasa City.
• A guy on Route 13 gives out free flutes (up to five total).
• Free musical props are available in Opelucid City.
• Cilan, Chili, and Cress offer a double battle.
• Based on the time of day, a lady on Route 8 gives out free rocks.

Just thought I'd add a few more things.

-Scientists in the Castelia Sewers can be visited daily to get items such as Revives or Super Potions.
-There is a different Pokemon Swarm everyday. You can find out where by looking at the scrolling text in the gates.
-There is a woman in Castelia City next to the Name Rater who will give massages to your Pokemon.
-You can battle your rival everyday in Driftveil City in the Plasma house.
-Every Thursday, a Mandibuzz or Braviary will appear in Route 4 (Depending on your version) and battle you.

I really only know of a few that haven't already been listed.
-The Game Freak people (2 instead of 1) can be battled every day inside of the 2nd floor of the Game Freak building.
-The Pansear Show (formerly Patrat) can be done inside of the old Striaton Gym for a Big Mushroom.
-Socialite Lady in the residential area of Humilau City tells you to walk her Mienfoo each day for a Pearl.

While not a daily event, N will ride the Ferris Wheel with you on Fridays.

If you finished the Yancy/Curtis sidequest, you can call them everyday and trade for a Pokemon with is Hidden Ability.