The breakfast street near the hotel is - find it on the first morning rather than the second because starting a Taichung day with actual local food sets a tone that hotel provisions alone don't quite manage. The options within walking distance are good enough to make the walk worth making before you've fully woken up, and that combination of early street food and city waking up around you is one of those specific pleasures that urban Taiwan does better than almost anywhere.
Toyo Ito's National Taichung Theater deserves a proper visit rather than a passing look - the interior spatial experience, the interior experience of the building I mean, is the part that photographs don't capture and which requires actually being inside it to understand what Ito was doing with continuous curved surfaces. Buy a ticket to something if you can. The public lobby is beofre the performance spaces what the entrance hall of a great cathedral is before the nave.
The coffee culture in Taichung is a genuine subject worth understanding before you arrive - the city has developed a specific Taiwanese approach to specialty coffee that's worth seeking out deliberately rather than - ask the hotel staff for a specific name and you'll get somewhere genuinely good faster than any app will take you. The concentration of serious independent shops in the area around the hotel is higher than most neighborhoods of this size.
Taichung's night markets are worth distinguishing between, the night markets here in the city I mean - the Fengjia Night Market is the large famous one and worth one visit for scale and atmosphere, but the smaller neighborhood markets in the eastern districts are where the better eating is and where the genuine night market culture lives rather than performs. Ask staff which one is worth the taxi ride this week.
The weather in Taichung, the weather in central Taiwan generally, is more variable than the island's reputation for warmth suggests - summer is hot and humid with occasional afternoon thunderstorms that arrive fast and leave fast, winter mild but genuinely cool in the evenings. The transition seasons are when the city is at its best and when the ambtiion of a proper visit, a first visit I mean, is most comfortably realized without fighting the climate. If you're timing a first visit and have any flexibility, October or March are the right answer and - anyone who's been to Taichung in both seasons and in the summer will tell you the difference is not subtle.