Math Grades 3-5
Problem Solving Decks - Deck B is best for this age.
Math Academy - 5 free units that help students apply math to the world around them. Include units on probability, patterns and functions, fractions, decimals, percentages, data collection and analysis, and more! Though they say for grades 4-8, I would recommend for 3rd to 4th grade gifted typically.
The Stock Market Game - All you have to do is sign up, and student can learn about stocks, bonds, mutual funds, diversification, and more while playing a simulation that mimics the Stock Market exactly! They get to compete with students across your district, region, and the entire US to see who can invest $100,000 the best. There are fall and spring sessions for competition, so sign up ahead of time, teach students a few lessons in advance so they know the basics, then start competing!
I made this slideshow for my 5th-8th graders based on the free lessons that The Stock Market Game provides. Hope it helps you to more quickly and easily implement this awesome resource!
Project M3 - I'd be crazy if I didn't mention this series for grade 3-6, but it comes at a steep price (about $300 per unit). High quality units with fun themes, but you'll need a decent budget for this one. Suggestion: Look up an older version on Amazon. The older version are nearly the same and cost around $50 instead.
Example: What Are Your Chances? was a great unit for my kids. We watched videos on carnival scam science, created our own games, hosted a "carnival", and saw who profited the most and why. You can pay $235 for this on Kendall Hunt or buy this version and the accompanying student book from Amazon for about $50.
Hands-On Equations: Great way to understand algebraic equations and fractions using hands-on activities. I recommend the Hands-On Equations Learning System for this age range.
Clark Creative Math 21st Century Projects - A teacher and student favorite is The Game of Life!