Which dungeon is that? I've just done the first one and done the quest to get the Mask of Scents (whatever that's used for). I'd imagine that going back will make the swamp poisoned again; so I've got another 16 hours or so (20 minutes?) to have a look around. There's a Skulltula house but that could be done during another run through no doubt.

Stone Temple, the fourth dungeon.
You use the Mask of Scent to search for mushrooms in the Mystery Woods to make blue potions with that witch. Not very useful.

Yeah, swamp will be poisoned again if you reset time, but you can teleport straight to the boss if you return to the dungeon, so it's no biggie.

Don't forget to do Koume's target game btw, it gives you a bottle which you will need later on in the Pirate's fortress (you need 4). You can only do it when the swamp is restored.

I've actually beaten it without the double magic meter before. Or Chateau Romani. Both of which I really recommend if you want to do it without trouble.

The greatest of all, DQ 9.

R.I.P Dragon Quest in the West.

Alright I'll do the target game now then, that's by the entrance to the swamp isn't it? 

Yep, it's quite easy.

I really want to get a decal for my 3DS, though I can't find one that I like .-.

all of my yes!

Good times.

Don't act like much changed.