Task cards get used in group work most often in my class, but they are perfectly suited to individual use. You can always direct early finishers to a set of task cards to work on. Just keep your collection sorted by grade or book, and let your students know to choose a deck from their area and just get to work. Your more advanced students can self-study with higher level cards. At the other end of the spectrum, lower level students can get remedial practice. If you keep the file of answer sheets handy, they will always know what they have already done.
If your school wants students to get to work as soon as they walk in the classroom, you can use task cards as a bell ringer activity. Either choose a random card and write the task on the white board or have each student draw a card from the deck as they enter the room. The task can be anything from a writing prompt for a quick write to a card (or cards) related to the previous lesson as a review.