Writing in salt (this site has Montessori Montessori-inspired activities).

Young & Lively Kindergarten: Here's another block center idea...

Activité Montessori

Counting and Measuring with Lego: Preschool Maths Game - Good way to visually compare numbers - I would suggest making the corresponding numbers (like 5 and 15 or 8 and 18) the same color to help see the pattern relationship between the numbers (5 and 15 or 8 and 18 are separated by 10)

Counting with Legos. I will make charts with multiples of 5, 10, 20, and 100 and use different colors to represent varying units.

Ijyy

Math- Addition Cups! Cups have holes in them so that the manipulative a drop through. The kids use white board markers to write the numbers.

Luukku.com

Math- Addition Cups! Cups have holes in them so that the manipulative a drop through. The kids use white board markers to write the numbers.