If you have access to any classroom magazines, reader's theater plays, or professional books with reproducibles in them, make good use of those!
I particularly like reader's theater plays because kids tend to enjoy the better written ones, they can read them as a class and then answer questions or write in a journal based on it.
If you can get a picture book and make a quick writing or language arts lesson based on it, it would be pretty simple to pull off.
If you're short on time, a lot of textbooks have supplemental material that is usually not used for regular lesson plans. The unit and chapter reviews, magazine excerpts and extra activities could be used for an emergency. Videos are another option.
As a side note in the emergency plans, if you have a well-practiced center routine or review game that students enjoy (for math facts, vocabulary, etc), then you can add that as an activity choice in the plans.