What do you have to do to get the additional xp option?

I don’t have extensive experience with EoD, but I don’t think there is one. City actions are unique to each city.

I might be wrong, not very familiar with Riverwatch.

Im trying to find it, but somewhere either here or BGG, there's a post with each city and how it gives XP. Might have to hunt it down.

There is no consistent 'gain XP' city action in Riverwatch.

If you do the Mercenary jobs in the Dented Buckler, at some point they will offer you a special mission, which you can do once only, which will give you 1 XP each at a cost...

If you refuse to beat up the baker then you won't get any more of the Mercenary jobs and you'll never get the extra XP mission.