Some important calculus concepts are not taught in lower level classes -- such as the concept of a limit. A good activity to start students thinking about limits is to start with a quantity and continue to cut it in half. Perform this thought experiment several ways; start with the number 100 and begin cutting it in half. Does it ever get to zero? Limits are something you can get as closer and closer to but never reach. Understanding limits will save a lot of time in the early stages of learning calculus.
Calculus requires a through knowledge of algebra. Two aspects of algebra that cause problems with students learning calculus are exponents and logarithms. A good activity for getting a feel for exponents is calling on students in class with exponents of a fixed number. For example, you can choose 100, and call on students to raise 100 to different exponents -- without calculators -- as you are trying to give them a feel for exponents. Start easy with exponents of 2, 3 and 4; be sure to include 0 and 1, as students often get these wrong. Then move on to fractional exponents. Do the same in-class exercises for logarithms. Choose a base and call out value to take the logarithms of. Be sure to try and trip them up by calling out negative values and values less than one.
The really important geometrical concept for precalculus is that area can be computed by slicing up a figure into simpler shapes and adding together the areas of the slices. A good activity is showing how the value of pi can be estimated by looking at a circle of known diameter with both an inscribed and circumscribed regular polygon. As the number of sides of the polygons increases, the estimations of pi get better and better. Show the students that the real value of pi will be found as a limit of the process.
The important points from trigonometry that are need to start studying calculus are the graphs of the sine and cosine, the sine and cosine in the unit circle and how to solve trig identities. Solving problems with triangles rarely comes up in a beginning calculus class, but the curves -- especially of the sine and cosine -- and the basic relationships between the trigonometric functions will come up often. Activities to refresh this knowledge will help prevent the student from being blindsided in calculus class.