I would highly recommend trying the Grab app instead of taking taxis. This can save you a lot of hassles (and money, especially if heading to/from the airport!)
Visit the Batu Caves as an easy day-trip if going to Kuala Lumpur! It's easy to reach by public transportation or taxi and well worth a visit. You climb a number of stairs next to a giant golden statue towards an entrance into the side of a mountain, then descend into a subterranean temple complex! Very memorable experience and too close to KL to pass up (about 14km away).