Create something that helps solve an inconvenience to you!
Example being, for me whenever I go to Wegmans I can't find what I need. Therefore, I went on their website and found out I can scrape the location / price of food per each of their stores. So I'm working on a little personal application that helps create a shopping list with the location in store and price of food that I want to buy, but I'm making it in a way so that it works for all of their stores just in case I decide to deploy it.
Simple things like that are awesome ideas. Don't go for ideas like creating a new social media app that will compete with Facebook or Instagram...I mean those are still good portfolio builders since it gives you an opportunity to fullstack, but don't really bring anything new and out there to the table.
You can also go to nearby businesses and ask if they need a website, or better yet don't even ask them and just create the website. Then when you get something nice looking you can walk in and let them know about it and see if they're interested in buying it or using it.