1. According to the weather forecast, it will be rainy. or "it's going to be/will be cloudy with a chance of rain," or "it's going to be a rainy day," or "there will be scattered storms."
2. There will be occasional rains tomorrow. (ok, but more naturally: "It's going to rain on and off tomorrow.")
3. It isn't (or: It's not) worth buying a lot of clothes today because there's going to be a huge sale in a month. (or: because everything will be on sale in a month.)
4. "It isn't worth going to the seaside because winter is going to be mild without any snow." I would say: "It isn't worth going to the seaside since it's going to be such a mild winter this year."
5. "The doctor was waiting in the wings since one of the actors was not feeling well." or "The doctor waited in the wings since one of the actors felt unwell."
6. There isn't an ocean, sea, or lake on Earth that isn't used as a waste dump.
7. You were only going to borrow this book for two days but you have already kept it for (two?) weeks.