Agreeing to go somewhere else with 賛成

Kanji Explanation: Becoming and accomplishing with 成

After moving and transferring schools, Lulu has finally made some friends in class. Following some after-school shopping, they stop by the group’s favorite café, only to find it filled to the brim with hoodlums.

響子(きょうこ):: 「残念(ざんねん)だけど　他(ほか)　行(い)きましょう」; “Too bad... let's just go somewhere else.”; Literal: “It's too bad, but let's go somewhere else.”
清美(きよみ):: 「賛成(さんせい)」; “Agreed.”; Literal: “Agreement.”
美代子(みよこ):: 「そうしましょう」; “Yeah, let's do that.”; Literal: “Let's do that.”

Key Points

1賛成(さんせい) works as a standalone I agree

By itself, 賛成(さんせい) means approval or agreement with the suggestion that was just made.
In dialogue like this, it is a brisk natural way to say “Agreed” or “Sounds good”.