。
Let's make it dinner.
Literal
Dinner [to-に] do-[polite-volitional-しましょう].
~にする ('to decide on X, to make it X') is the decision-making frame: deciding among options, 'let's go with X.' Here ~にしましょう is the polite volitional version — 'let's make it [dinner].' Useful at a restaurant when choosing what to order or how to spend time: 夕食にしましょう could mean 'let's go to dinner' or 'let's call it dinner time.'