Math Grades 1-2
3 Act Math Tasks - Problem-based lessons that will foster students' curiosity by having them dive deeper into the problem with 3 related tasks.
Project M2 - I'd be crazy if I didn't mention this series for grade K-2, but it comes at a steep price (about $150 per unit). High quality units with fun themes, but you'll need a decent budget for this one.
Hands-On Equations: Great way to understand algebraic equations and fractions using hands-on activities. I recommend Early Algebra for this age range.