Our beach plans completely depend on the unpredictable weather this weekend.

The verb "depend" pairs with the preposition "on" (never "of" or "in").

I totally agree with you that we need a solid backup plan.

When you share the same opinion as a person, you "agree with" them.

I will think about some fun indoor activities just in case it rains!

We use "think about" when considering an idea, a person, or a possibility.