For me, I always have to give my characters an interesting quirk that amuses me to no end no matter how many times they do it, whether it be drinking all the mead they find, or collecting skulls for their personal collection. I also tend to do more exploring, and make up things they might be thinking in different situations. But, that's just how I stay with one character long term, which doesn't necessarily equate to high levels.
My bard, for example, uses no melee weapons, and relied on her fists, with no armor. I think at the moment, her main skills are archery and sneak, as well as speech, but other than that, she's been on level 8 for a long while, but I've had fun with her, especially with turning Windhelm into a forest of impaled, burning corpses kindly donated by the local guard garrison, which would be her little quirk... or maybe a skeleton in the closet she's kept from her spouse. But, my main point is to give a little individuality to your characters.