Start from a real world scenario that you know and you are familiar with
Faking stuff is hard, better to stick with what you know and experience directly: take the data from your personal or work life and use to create the canvas for your demo dataset, next you'll replace names and other details.
Make it familiar for your audience
Who is your audience? Are they young? Is gender relevant? You probably already have a good idea about that if your are building a product, keep this in mind when choosing the actors in your dataset.
Keep it as straightforward as possible
The goal of your demo dataset is to showcase your product, not your demo dataset, try to make evident what is what: for example, use well known identifiers for companies (LLC, Inc, ...), locations (City, Lake, ... ), projects, etc.
About using existing brands
Trademarks are a complicated issue, it may be fine to use existing brands or it may not, treat with care. When not sure, don't: better to avoid to recreate your demo and material because of trademark infringement.
The holy grail: tailored demo data
Can you think of a way of generating tailored demo data? maybe processing publicly available info from Wikipedia, GitHub or Crunchbase? Then go for it, showcasing a product using your client data will be way more effective than using unknown or fake data.
What about well known dataset?
It depends, well known for who? If your are not 100% sure that your audience is familiar with a topic is safer to assume that they are not.
And since your are here, check out VALIS!
it doesn't apply for every type of project 😉 ↩