private void dataGridViewInOut_CellEnter(object sender, DataGridViewCellEventArgs e)
{
    DataGridViewComboBoxCell combo_cell = new DataGridViewComboBoxCell();
    combo_cell.Items.Add("one");
    combo_cell.Items.Add("two");
    dataGridViewInOut[e.ColumnIndex, e.RowIndex] = combo_cell;
}