Daily challenges seem kinda redundant. Why not spice them up and make them something that forces you to go do something.
Get 10 spar wins, Get 20 pks
Kill 1 of each baddy type, Kill a gold baddy
Win a carnival event, Help capture a tower
Visit (some random npc), get a kill with a bomb explosion,
Get a kill with an arrow, play a game of archery
Lots of simple tasks you could implement.
And then you could get into much more advanced challenges.
Go on a 10pk streak, Go on a 5 spar streak
Kill a 2 lizardons without taking damage
Stay in a pk zone without taking damage for 10 minutes
Take a bush from northern wilderness to bazaar without uncrouching or dropping it.
Get a score of 8000 or higher in an archery