Another start-up I've been playing with is Railway, who offers 5-10$ of free usage per month, certainly enough to play with react/nextjs app and a postgres db to your hobbyists hearts content (as long as you turn if off when you're done).
If I were to host a bootcamp on starting a web app from scratch I'd do something like stand-up a T3 App https://github.com/t3-oss/create-t3-app on Vercel Hobby https://www.vercel.com . Not sure I'd even consider Heroku for teaching anymore.