I think the hardest part of "More Kids" is finding MORE HELP. Train your CE committee to poke their heads into workshop and see if the teacher needs help on Sundays where it's obvious we have a lot of kids.
I think "what to do" depends on whether or not the attendance fluctuations are predictable, such as, during Holy Week, or unpredictable. "What you do" also depends on which workshop you're in when the numbers arrive.
Art, Music, Video, and Games are usually easily adaptable to the sudden influx of unexpected numbers. If you know in advance, all the better.
However, Cooking, Computers, Science, and Drama are much harder to quickly adapt to sudden influxes of students. They are more number sensitive.
Almost all workshops can be scaled down when you have unpredictably few in number. The only one which can give you trouble with a sudden dip in attd is Games, --if your game requires a minimum number of kids to make it fun.
I like to have a VIDEO hanging around that I can pop into the DVR or computer for those times I need to punt. If I'm in the computer lab (which often is the case) and we get slammed with too many kids, I'll turn on a Bible trivia game, such as, Fall of Jericho CD.
If you can predict SEVERAL WEEKS of abnormally high attendance, then I suggest you ADD a workshop and split some kids if you can. Instead of four workshops for 50 kids, do five workshops during that time to keep class sizes reasonable.
OR... do NOT schedule certain workshops that are hard to scale up. See my list above of those workshops which are harder to scale up.
Look for Workshop Lessons here at rotation.org that fit your story and will work with LARGER groups.