On the logistical front, I bring the base set plus two expansions (counting Alchemy, Cornucopia, and Guilds as half-sized) to the table. That usually gives a nice amount of variety without too much effort to carry. Sometimes the randomizer doesn't pick cards from one of the expansions, but that's randomness for you.
When I started with my regular group, we did recommended sets for quite a while before switching to random. Either approach works. Since you mentioned Dark Ages, perhaps consider a kingdom of Rats plus 9 random cards, since it's Donald X.'s favorite card. Showcase the pinnacle of Dominion card design!
Probably best to warn your friend that Rats must be handled with care, else it will eat their deck, though. If you're interested in recommended sets, one of the most memorably enjoyable I've tried was Infestations (Dark Ages + Alchemy).
You asked about the Travellers in Adventures - both lines are quite good. The Peasant line might be a good choice for strategizing, since it can be non-obvious whether to promote your first Disciple to Teacher or keep it as Disciple. Though maybe the slower pace of 2-player makes Teacher usually correct, I'm not sure.
Adventures and Dark Ages are my two favorite expansions, but they're all good, so you should be fine whatever you choose to do.