I've become bored with combat. Everything is too easy. The only challenges I have are Briarhearts and Ancient Dragons, but even they're easy if I have enough potions. My Smithing and Enchanting are both 100, though my Daedric armor hasn't been upgraded or enchanted. I have no perks put into either Heavy or Light armor. My Daedric bow hasn't been upgraded either, and I usually use Orcish or Steel arrows.

What can I do to challenge myself other than running around in robes and using an iron dagger?

Same thing happened to me, my character just kept growing stronger and stronger until I wasn't matched.
So I decided to make an unarmed fighter which was fun.
Just grab the gloves of pugilist from the fist fighter in the ratway then grab the perk fists of steel in the heavy armor tree.
Spices things up for a while.

Are you playing on master difficulty? If so, you can try going through the game without using smithing, enchanting, or alchemy or at the very least, minimize their use to rpg-uses instead of power leveling them to max.

These threads are starting to get old.

Make a new character, play on Master difficulty, and don't use Smithing or Enchanting. And play naked, fisticuffs only.