A niche should be focused so google sees you as an authority and will send you traffic. So if you have a travel niche try to focus on one element that you can discuss regularly for years to come. So you might decide that you want to discuss travel gadgets like pillows, bags, etc. But it's all under the topic of travel gadgets. So think like that. The same applies for makeup. You may want to have a make up blog that just focuses on eye makeup. Hopefully this is giving you ideas of to narrow topics down. You just want to avoid being all over the place or too general.

You have to be disciplined enough to stay within a more focused area of your niche. That's what Google rewards especially when you're a new site.

My niche isn't on traveling. Asking WAers seems like a great way to get a wealth of information on food from everywhere.