You want to focus on number theory, not math. Think about things like "this stack has more blocks than that stack" or "let's make stacks of the same size".

You could make 2 equal groups of blocks (e.g. 3 each), then move a block from A to B and show that B has more blocks now. Or create 2 unequal groups of blocks and have them add blocks to make them equal.

Or have containers with different quantities in them, or of different sizes, and have them arrange them in order.

That will be plenty of effort for just 1wk with a 18mo child. I have a child of the same age - they have amazing interest in things around them and very good memory, but teaching concepts takes focus and attention which they don't have as much of.

As others have said, at this age just playing and exploring (and love/attention) is the best thing you can do for them. Studies have shown that math can easily be picked up much later (age 4-5) without detriment.