For the payment of land purchased for parks or boulevards, for the building, maintaining, improving and protecting of the same and for the payment of the expenses incident thereto, or for the acquisition of real estate and lands to be used as a site for recreation centers, any park district is authorized to issue the bonds of such park district and pledge its property and credit therefor to an amount including existing indebtedness of such district so that the aggregate indebtedness of such district shall not exceed 21/2 percent of the value of the taxable property therein, to be ascertained by the last assessment for county taxes previous to the issue from time to time of such bonds.