I'm not sure why you're using ActiveCell as your starting position, but I'll
stay with it.

You don't say what the formula  is. In particular, you don't say whether it
is to be relative to where it is located. I'll assume it is to be relative.

An easy way to put a relative formula in is with FormulaR1C1. This
statement puts a formula in the cell to the right of the active cell and
every cell below it until the next blank cell.

Range(ActiveCell.Offset(0, 1), ActiveCell.End(xlDown).Offset(0,
1)).FormulaR1C1 = "=RC[-1]*10"

This formula is multiplying the cell to the left of it by 10.