My group botched claiming Hazirawn (never even knew it was there), so I put it in Arauthator's hoard instead. I also put a matched set of Swords of Dragon-Slaying (dagger, rapier, shortsword, longsword, all in a nice ornamental carry case) in the safest place the dragon could think of: buried in the ice underneath his bed. You could replace a gem with the carry case.Also, did anyone add any magic items to Arauthator's hoard? I'm toying with the idea of including one or two items. I'm also thinking I'll convert some of the gems into art objects.
Maccath has been stuck with the dragon for years, so I don't see cost as a problem; she's BORED. She also wants a way to slyly get revenge on him via a third party.
While they are in the boat, have the group meet a Plesiosaur who also has the Swallow Whole ability, and see if anybody recognizes the Loch Ness Monster.
(In my group's case, 90% of the sailors fell overboard but only 2 heroes fell joined them in the drink. Go figure.)