When talking about exciting elementary programs one should never omit CSMP, R(not to be confused with ucsmp)
Every lesson in csmp is replete with more mathematics than most high school "memorize the formula and plug in the numbers" lessons.
For example, the very first lesson if first grade has children exploring relationships between two collections (mice and cats), using concepts (but not the language) of belongingness, inequality, empty set, quantity. Later lessons that same year (i'm talking about 5-6 year olds) have learners exploring concepts very directly dealing with (again, without our sophisticated language) negative numbers. A lesson i particularly like, and this, too, is first grade, has ten children putting first one, then the second puts three, the third puts 5, the fourth puts 7, up to the tenth who puts 19 pennies into a bag. then the class estimates (or guesses) how many pennies, then calculates how many pennies using an abacus type of device, and finally explore the number sentence 1+3+5+7+9+11+13+15+17+19 for patterns that might help them determine the sum differently. That's after they have already found it from the abacus.
What do these students do in fifth grade? i'm glad you asked.