If you had to pick the one grammar point that makes a classroom liveliest, it's can. The topic forces children to talk about something they're proud of, which is what they can do. I can swim! I can ride a bike! The structure is generous too. Can looks the same for every subject, there's nothing to conjugate and no exceptions to warn anyone about, so children get it right from their very first sentence.
Open with Actions, Not the Textbook
Say I can clap! and clap. Then challenge the room: Can you clap? They clap and answer Yes, I can! Keep going with jump, sing, dance, touch my nose. Within five minutes the whole class has produced the statement, the question and the answer, and nobody has opened a book. This is grammar that teaches itself through the body.
Next comes cannot. Say I can swim, but I cannot fly! and flap your arms in a hopeless attempt at take off. The laugh you get is your evidence that the meaning has landed.
The Writing Task: An I Can Card
Hand out a sheet of paper folded in half. On the left they draw and write three things they can do. On the right goes one I cannot... sentence for something they wish they could. Finish with half a minute each at the front of the room. Shy children are far braver when they have a piece of paper to hold on to. The finished cards will fill your back wall for the whole term, and parents love them on visiting day.