Some Montessori activities are genuinely complex, and can be very confusing for adults unaccustomed to our approach. However, some other Montessori activities are very simple to understand, but can breed endless complexity with little tweaks.
One of our favourite introductory practical life exercises is the lock-and-key set: this is a simple set of locks and keys, with children trying to match the correct pairs. Based on the child's level, the set can contain a variety of differently sized locks, or a larger number of pairs.
It is not unusual for us to see children younger than 3 working on solving this puzzle for an extended duration of time without asking for any help. It is indeed a wonderful activity for developing fine motor skills and problem-solving abilities, and very easy to replicate at home!