Jokes aside what is your one day? 24 hours? Check in 3PM on day one check out 10am the next day? Or check in 7PM, stay one day, then check out 10am on the third day?
If you're there for just a rest, Osu shopping district is good, it's like the default shopping area for everyone. Not like our orchard road which is only for the rich. You want food got food, want souvenir got souvenir, want camera got camera apparently. Want anime stuff got anime stuff, the Lashinbang there is good.
If you got one full day, the abovementioned maglev and Toyota museum. And maybe some more stuff I guess like aquarium, I need to read the Yatogame anime/manga to remember.
More importantly, go Nagoya is to enjoy the weird food. Chicken wing, miso katsu, weird ice cream, weird snacks, weird drinks. Sugakiya branches everywhere, if you're lucky you might even encounter one that sells the weird spoon/fork/spork. (I got mine at the basement food street to the west of Nagoya station) Komeda and other cafe famous for their "morning set" where you buy coffee/tea/drink and comes with toast with ogura or egg. Tip: The branches near tourist areas will long queue, but the branches at normal citizen areas will have seats.
Osu + Nagoya station can cover your shopping and food needs. West of Nagoya station also got anime stuff.
And watch the Yatogame anime before going
