Mr Money Mustache [1] advocates just investing in index funds. Others push things like real estate. Really depends on where you interests and talents lie.

A common thread that these people tend to find though is that once you've saved up that much money, what you have actually learned is how to efficiently grow wealth, and I have seen very few examples of people actually just stopping there and coasting. Usually they just keep on trucking, maybe shifting into a different field.

[1] https://www.mrmoneymustache.com/