extra activities
If you sub enough, there will likely be situations where you may need extra work. I subbed for a teacher that put together really nice lesson plans. Unfortunately, she cut and paste the plans from the previous day's sub plans. She managed to leave me plans with only two subjects updated. I sub for another teacher who is the Queen of Worksheets. Every time there is a sub, she leaves worksheets for the entire day. For some kids this is great, but for the ones who are a little more attention deficit, it's BORING!! Other times you are given lesson plans that just aren't meaty enough.
My first choice is usually to have the kids read silently for a few minutes. Not too long, or they'll probably get unruly. 15 minutes is a lot for elementary age when a sub is present, not sure about older classes. Depending on how much time we have, or how beneficial the lessons left were to the children, I sometimes ask if there is a "class book" the teacher is reading aloud. The kids usually like for me to read a chapter of whatever book the teacher has been reading to them. The teachers that read aloud to their students usually choose books that most kids like and they hate to miss a day. Of course, those teachers will usually give a read aloud time in the schedule, but in that case, you can pick an extra time to read that extra chapter. I like to carry flashcards appropriate to the grade I am teaching that day. If we have extra time, we can always play around the world. You can ask a student for a copy of the current spelling words and hold a spelling bee or play travel with the spelling words.
In the case where the teacher hasn't left lesson plans, you could definitely go to the team teachers for that grade. Also, in our school, we have IST's that address any concerns with the lesson plans. I have subbed for a teacher who didn't leave lesson plans and the IST came in and made up lessons very quickly. If you have the teacher manuals, you could also pull a student aside and ask if they have done certain lessons. Otherwise, if the students have practice books, you can check their practice books to see where they left off.
Lots of solutions if you think clearly and don't panic.