XP - or rather, skill/attribute/talent points - should only be rewarded for completing quests, no matter how the player completed them. A character shouldn't get stronger than average just because they picked every optional locked door and smashed every skippable rat on the way there.
XP is mostly superfluous. Why do you lockpick a chest? To get the contents inside. Why do you kill an enemy? To get his stuff, make someone happy, or simply to get past him. Why do you talk a hostile character down? To keep him alive or avoid having to battle him.
Deus Ex did it well (with the exception of the minor exploration XP). It doesn't matter how you get through the level, you get the same amount of skill points. You make choices for the consequences, pick chests to get the contents within and kill enemies who are in the way. The new Deus Ex games give XP for all those things and it just makes it worse, arbitrarily rewarding the player more for playing a certain way and interacting with every minor inconsequential object.
Ending up with different amounts of XP at the same point in the story isn't interesting. What's interesting is the attributes/skills/talents chosen, the choices you've made in the world, the alliances you've forged, what equipment you have. That's what should define you.
How would taking quest to delliver potatoes to a local official make a character less likely to be killed by a single blow by a dragon? With high level characters we see that when dragon attack connects and they are hit, they typically survive. With low level characters and no level soldiers, they are killed and changed into flat surface.
That's a problem with progression being vertical instead of horizontal. A humanoid should always be one-hitted by a dragon, regardless of how trained he is. To fight a dragon you should instead poison the dragon to make it move slower, train agility to avoid its strikes, wear fireproof armor to endure the heat etc. Every little trick to even the odds.
Obviously with killing stuff, there can be excuse of main characters eating souls. Or using black magic to strengthen theirs bodies, and theirs opponents are just sacrifices.
There is no such occurance with a potato delivery quest.
Thus why would a character in RPG get XP for mundane activity?
If your quest is , say, to bring peace and order to a region, the XP should be rewarded only upon completion of that goal.
Helping out a farmer by delivering potatoes could net you a friendly farmer who knows a secret way into the Lord's castle. Or it's very early game and the pitchfork you get as a reward might be your first weapon that deals piercing damage.