Every teacher has a few "sponge activities" ready for action, whether it be the first few minutes while attendance is taken, or the last few minutes of class at the end of a lesson. After all, "the best laid plans of mice and men often go astray"...What do you have up your sleeve?
I know journals are very common, but I like to use them as opportunities for students to build writing fluency and also practice writing essays. I choose topics that I might normally ask students to write essays on, and I have them write mini essays of 3-4 paragraphs in 10-15 minutes while in class. I have found that by doing this, students no longer gripe about writing and are able to write more and faster, and with better organization. I give them time after writing to discuss what they wrote in partners or groups, which is its own reinforcement.
I love to use famous and/or thought provoking quotations as a springboard for writing. These can make great sponge activities and also fantastic advance organizers before reading a novel if the quotation ties in with the plot,, theme, or characters in some way.
I do something similar with quizzes sometimes as well. I will pull a line or two from the previous night's reading and ask students to explain how that quotation is relevant to the chapter they just finished reading. (Of course, they have to use enough detail to also show me that they read the chapter in their response as well.) Such a short writing activity often makes a great lead-in to insightful class discussions.
One of my favorite sponge activities was to have students react in writing to a quotation I had put on the chalkboard. The three to five minute freewrite not only made for a quick sponge activity that allowed to to tackle other administrative tasks, but asking for volunteers to share the ideas they wrote about often led to fascinating discussions and insight that often tied in with the material we were currently studying.
Lateral thinking activities are always good, but my freshmen and sophomores love language activities best. For example silly sentences: Students are given seven letters on the board, and they have to make a complete sentence in which each word begins with a letter on the board. The hard part is the words have to come in order left to right or right to left, they have to make sense (although they can be silly), and they must be complete sentences. You would be surprised at how many students recognize and call out to their classmates problems with the sentences.
When I was in the classroom, I kept a box of Trivial Pursuit questions in my desk precisely for the type of situation you describe. I also used the "One-Minute Paper" (or "Three" or "Five-") as a final check for understanding. I'd ask the kids to do a quick timed freewrite and let me know if there was anything they did not understand about what we had done in class that day. If they did understand everything, then they were to write me at least one tip on how to make that lesson more engaging for the next class I did it for.
I keep a book of puzzles and brain teasers to pop up on the board with the document projector. I also like the word of the day which includes prefixes, suffixes, and root words...this quick and easy exercise turns into an end of the week or bimonthly vocabulary BINGO which does help with word acquisition and ACT/SAT prep.
Word of the day is my 'filler'. Students are invited to 'collect' new words for us to share, but I also use Merriam Webster's online page if occasion calls- they have great hangman activities there too.
I do a competition where I write a 10 letter word on the board and students have to make as many 3 letter words and above out of it as possible. We then go through and cross off any ones we all have, each student getting 1 point for each word they find that no one else has discovered.
I have some very "fancy" connect the dots activities that older students like to tackle. Like the previous responders said, I also have many crosswords, brain teasers, etc. I usually let the students pick from specific things that I have set aside for extra time. That way they can pick something that they are interested in.
Trivia questions, brain teasers, that kind of thing; keep a box of some sort. When I have some kids that finish things much faster than others, I have printed out some Sodoku, or crossword puzzles.
Riddles. When we finish early (or often because students rush to finish early) I give them a riddle - or if I have a short lesson that doesn't leave quite enough time to move on.
I also like assigning ongoing projects and "scheduling" down-time for kids to work on them.
I also do 45 min of silent reading every Friday (on block schedule). After the first few weeks of class - I actually see kids reading their independent novel when they finish with something early - which is pretty awesome in high school.
I always keep a list of writing prompts for short writing assignments handy. I try to bring a newspaper to class to discuss any interesting current news. I always keep a quality short, short story in reserve in case of emergencies--such as a malfunctioning copier or shortened periods due to unexpected assemblies or fire drills and such.