What kind of hotel can I let in without an ID card? I mean, you can tell by the brand, or imply that I'm not 18 years old.

If you need an ID card in a bigger hotel, you can go to a small hotel. Those hotels are generally not very formal and generally do not need documents. And those hotels generally don't want documents unless you take the initiative.

My friend stayed in a small hotel for two days, under 18, without an ID card. It's up to you.