I always find that the best way to begin introducing new materials to children is to place them in a basket or a container on one of the tables and simply allow the children to explore them independently for a few days. During this time, you can observe how they approach the materials without any instruction.
After the children have had ample time to explore the materials independently, you can begin introducing simple activities that explore mathematical concepts with them. Unifix cubes are one of those materials that children will play with independently for ages. They provide endless possibilities for young creative minds. That said, they are also designed to support many mathematical concepts including counting, patterning, sequencing, grouping, sorting, etc.
One way that children love to use Unifix cubes is to see how long they can make them. They put them together until there are none left in the basket. They want to see if they will go from one end of the room to the other, or from one end of the rug to the other. Even though this may seem unrelated to math, this is a great opportunity to talk about length (longer and shorter) and quantity (a lot, a little) .