My experience with dialog flow is minimal. I want to add clickable button responses that an end-user can select from. I do not want to use any other platform except for Default. I will be implementing this on my website. I have not enabled webhook and fulfillment yet. I just want to write the code to add buttons, test it, and then publish my agent on the website.
I have bad news for you. From the default features the only option you have for implementing it on your website is using Web Demo which does not support rich responses(button, image, etc..). That means if you want to implement a bot with rich response on your website, you will have to enable Fulfillment and use webhook.