What type of tree is 香榭?
香榭 refers to 香榭丽舍大道, i.e. Champs Elysees of Paris.

礼物不需挑最贵 =&gt; Don't need to choose the most expensive present,
只要香榭的落叶 =&gt; just want the fallen leaves from (the trees at) Champs Elysees

BTW, even though fallen leaves are free, money is still required for the air ticket and hotel to go to Paris. (which should be what OP wants indeed)
And, you can buy 香榭的落叶 at 淘宝. (which shouldn't be what OP wants)
See 七夕快了，来自香榭丽舍的落叶居然成了告白神器.