Online videos: Khan Academy has some great resources. Check out their Arithmetic section.
Online game: This is a fun "constructions" game. Maybe a bit advanced, but is pretty coo, nonetheless.
Some general ideas:
Give the kid a jar of pennies, and have him/her make patterns (e.g. rectangles, squares, etc). Start it off with supervision/guidance, and then let them play on their own. Have them count the pennies, first by ones, then by twos, etc. Show how grouping the pennies in stacks of $5$ can make it easier to count without mistakes.
Put up a "hundred chart" in their bedroom (or somewhere they spend a lot of time). Show them patterns like how the multiples of $5$ are in two columns, the multiples of $2$ are in diagonals, the multiples of $9$ are also on diagonals. Show them the Sieve of Eratosthenes to find "prime numbers" when they understand multiplication.
As soon as they know of a multiplication table, show how it can be used to find the number of pennies in a $8\times12$ rectangle.
Introduce them to "recreational math" problems There are plenty here under the recreational-mathematics ranging from super easy to super hard.
Sure: many of these involve some parent (or other adult) help. But that's just because there's no substitute for a parent/significant adult who cares. If the parent takes time out of their busy schedule to (patiently) work with the kid on math, it sends a message: "Math is important to me (the parent), and I care about helping you succeed." Kids (especially young ones) care about what their parents care about.