How to say "we're done for the day"?

今天就到这里吧
Jīntiān jiù dào zhèlǐ ba
or you may hear it like this
今天就到这儿吧
Jīntiān jiù dào zhè'er ba

Let's call it a day / That's all for today.

Can be used in any situation, doesn't have to be end of lesson, can be end of work, end of a sport session or end of a gaming session. Let's call it a day / That's all for today.

wǒ men jīn tiān de kè jiù shàng dào zhè lǐ
我 们 今 天 的 课 就 上 到 这 里。

Yes, and also we can say like this, "今天的课结束了" (jīn tiān de kè jié shù le).

Here's a breakdown of the sentence:

今天 (jīn tiān) means "today"
的 (de) is a possessive particle that connects "today" with "lesson" to indicate that it's "today's lesson"
课 (kè) means "lesson"
结束了 (jié shù le) means "is finished" or "has ended"

So, altogether, the sentence means "Today's lesson is finished."

今天的课到此结束 (or 到此为止)。