Could I arrange random gifts for the participants in my research?
Questionnaire design is a science.

Within that science, there is a body of literature on the ethics of incentives, and on maximising response rates. Behavioural economists and other social scientists have put a lot of work into this: you just need to make best use of their findings.

Just because questionnaire design looks easy, and lots of people do it shoddily, doesn't mean you should. Apply to questionnaire- and incentive-design the same academic rigour that you will apply to the analysis of the results.

In other words: the answer to your question is in the literature, so consult it: and then you've got some proper peer-reviewed literature to cite in your methodology section. And discuss the ethics with your university's committee and with your more experienced colleagues.