We surveyed 300 young people on what they like to do at week

句子不完整,in their后面有东西,可能是in their spare time.
主句为we surveyed 300 young people；on在这里相当于about关于；
what they like to do at weekends. what they like to do in their spare time.在句中充当名词的作用.
大概意思：we surveyed people about something
我们调查了300个年轻人,调查他们周末或闲暇时做什么.

此句未完。
我们对300名年轻人进行了调查，调查他们在周末或在他们的……喜欢做些什么。