Fresh seafood comes to mind. At least in Phoenix, it's flown in daily at great expense. Other highly perishable luxury foodstuffs would similarly follow, I'd imagine.
You're looking for something that goes bad in less than 1.5-two hours. Say it takes 30 minutes to get to the hyperloop depot, 30 mins to Vegas, then 30 minutes from the Vegas hyperloop dept to get to the final destination casino or wherever its going.
And it needs to be something numerous enough that you could consistently fill a shipping container with it.
I don't think the target is "goes bad" as much as "degrades in quality to a perceptible–or marketable–degree". Oysters, crab, et cetera (not sure about lobster) taste different, in New York, between the restaurants that make daily fish-market visits and those that skimp to three (or fewer) times a week.
The "fresh sushi" is more marketing than palate based - so say sushi/sashimi lovers.