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WORDS:
пойти (to go)
обедать (to have dinner)
читать (to read)
сесть (to sit down)
говорить (to talk)

If you would use ты to speak to someone use давай in these constructs and if you would normally use вы, then use давайте.

BTW, in the sentence, Давайте сядем на автобус, сядем is the future plural perfective form of садиться and so the literal meaning of the sentence is something like Let's sit down on the bus.

Давай обедать.
(Let's have dinner.)

Давай читать вместе.
(Let's read together.)

Давайте сядем на автобус.
(Let's take the bus.)

Давайте не будем говорить об этом.
(Let's not talk about it.)