They solved the puzzle! On the second attempt, with one additional Int check required to assist. There was some good conversation around the logic of it, with competing theories, and only two people took damage when their first attempt backfired.
Past that blockage, they did some light RP with someone who'd been trapped in the caves, the bard finally getting to Charm Person on someone in a useful way, and he led them through some exploration/skill check stuff. They fought a couple of bulettes, in a fight that's maybe the best-balanced I've created, using environmental stuff and random items found earlier in the adventure in creative and exciting ways – mostly successfully!
Then a young green dragon showed up and the Bard tried to soothe it, but failed pretty badly and ended up getting poisoned a couple of times before she decided that climbing onto its back would improve her chances of making friends. (The Cleric, Fighter and Rogue had other ideas, and killed it. Bard and Cleric have a grudge now.)
They'd only been hired to clear out the wildlife, but the trapped guy they found earlier had said some intriguing things about what was really at the bottom of the mine – not just the iron vein that they'd been told about – so more exploration was in order, and the session ended as the Cleric opened a magic door in some kind of polished stone wall that had been partially excavated by the miners.
I had to end it there because I've not actually written anything about what's inside this door yet; in fact, the idea of having something related to the main plotline only occurred to me the morning of the session. I'm trying to figure out the structure of it, but I think more combat's pretty much off the table. They've had a couple of challenging fights and are running low on dailies, and I don't want to have three combat-heavy sessions in a row because at least one of the players isn't so interested in that stuff (hence, attempting to befriend the dragon).
I've got a Rogue who's not really been so engaged with things, so I'm thinking about throwing some traps and acrobatics-y stuff at them. The Bard has fire resistance so I'd like to do something with that as well.
I probably will have one more fight before they get back to civilisation, though – the guy they rescued was pretty adamant that whatever is in the mine is his, so I expect he'll be waiting for them when they come out – or he'll follow them in and trigger some kind of self-destruct sequence in an "if I can't have it, no-one can!" moment.