A weather-dependent plan needs more than "We will see what happens." Try, "If the weather is suitable, we will have lunch in the park. If it rains, we will use the cafe beside the station." The alternatives are explicit.
Add when and how the choice will be made: "I will check the forecast on Saturday morning and message the group by nine." Other people now know when to expect a decision rather than waiting indefinitely.
Keep conditions practical
For grammar practice, "If it rains, we will go inside" shows a common future condition using a present form after "if." Other conditional patterns exist, so learn this sentence for its particular meaning rather than assuming every sentence containing "if" follows one rule.
The backup must be workable. In a real plan, check opening hours, access, and any booking requirement before announcing a cafe as guaranteed space for a large group. Language clarity cannot compensate for an unavailable alternative.
Role-play a new development: the cafe is closed. Revise the backup and say which earlier instruction is replaced. Ask your partner to explain where they would go in each case and when they would check their phone. If the answer is uncertain, make the decision point or communication responsibility clearer before considering the plan settled.