We only have 1 and a half days in Suzhou. Should we use all our time to explore this city?

Yes, you can use the time to explore the beautiful city!
I’d like to give you some advice for the visit! You can’t miss those classical gardens like Humble Administrator’s Garden, Lion Grove Gardem and Lingering Garden. Besides, you can go to Tiger Hill, Suzhou Museum, Shantang Street and Hanshan Temple!
Enjoy your time!

Will we regret not going to Tongli as well?

If you are interested in the water town, Tongli is a must-go. Besides the gardens, your time cane be enough to tour around Tongli.