Excel export would be tougher (there is a lot of formatting that goes along with it). On the simpler side... you can create it as a CSV (comma separated value file).
StringBuilder sb = new StringBuilder();
string[] columnNames = dt.Columns.Cast<DataColumn>().Select(column => column.ColumnName).ToArray();
sb.AppendLine(string.Join(",", columnNames));
foreach (DataRow row in dt.Rows)
{
string[] fields = row.ItemArray.Select(field => field.ToString()).ToArray();
sb.AppendLine(string.Join(",", fields));
}
File.WriteAllText("output.csv", sb.ToString());