One of the classic Montessori materials is the knobbed cylinders [sometimes called cylinder blocks]. They can be quite expensive to buy, but I found this mini set for a reasonable price and snatched them up.
L has mastered each knobbed cylinder set individually [for instructions on how to present them, look here], so I knew she was ready for an extension of the work.
I put all four of the knobbed cylinders on a tray and put all of the cylinders into a bag in the middle. I showed L how to take a cylinder from the bag and put it in the correct hole. This work takes a while to complete since there are so many cylinders and holes to choose from — this builds concentration and focus.
L was pretty excited to have completed the whole work! The knobbed cylinders are a sensorial material that teach visual discrimination of size and promote fine motor skills, pincer grip, and hand-eye coordination. For older children [or if your child is ready for a more complicated extension], you can try this activity while blindfolded.