The current trend may be to drop managed mutual funds, because between active management and fees they're not great on returns, but really stock picking is both

The current trend may be to drop managed mutual funds, because between active management and fees they're not great on returns, but really stock picking is both time consuming and risk fraught - most investing advice I see these days is to pick an index fund, or index ETF. There's now a diversity of indexes, and that's where the diversification comes from. You can hold an ETF as long as you please, or as short as you please, they're traded exactly like stocks. The only caveat is that leveraged ETFs really are intra-day holdings only, because they can do very strange (and bad) things if you hold them long term.