This may not be an all-inclusive list.

Changing between base five and base two to base ten
Expanded notation, place value
Properties of whole numbers including order of operations
Connecting addition, subtraction, multiplication and division using manipulatives, grids, etc.
Explaining methods like "take away," comparison, partitive, measurement, rectangular arrays, repeated subtraction, etc.
Understanding how the Latin-Euro method works
Estimation:  rounding, clustering, front-end 
Notation and terminology for divisibility, factors, multiples, etc.
Divisibility rules for 2, 3, 4, 5, 6, 9 and 10
Connecting factors and multiples to grid-paper and Cuisenaire rods
Prime, composite and square numbers and connections to grid-paper and Cuisenaire rods
Finding the prime factorization of a number (including understanding how the "ladder" method works), knowing when a number is determined to be prime
Finding GCF and LCM using Cuisenaire rods
Finding GCF and LCM using prime factorization
Understanding "relatively prime"
Operations with two-color counters - modeling and explaining
Understanding parts of a whole, shaded, unshaded, equivalent fractions using manipulatives
Understanding connections of relatively prime, GCF and LCM to fractions