You mentioned three concepts: (1) trading (2) diversification (3) buy and hold.
Trading with any frequency is for people who want to manage their investments as a hobby or profession. You do not seem to be in that category.
Diversification is a critical element of any investment strategy. No matter what you do, you should be diversified. All the way would be best (this means owning at least some of every asset out there). The usual way to do this is to own a mutual or index fund. Or several. These funds own hundreds or thousands of stocks, so that buying the fund instantly diversifies you.
Buy and hold is the only reasonable approach to a portfolio for someone who is not interested in spending a lot of time managing it. There's no reason to think a buy-and-hold portfolio will underperform a typical traded portfolio, nor that the gains will come later. It's the assets in the portfolio that determine how aggressive/risky it is, not the frequency with which it is traded.
This isn't really a site for specific recommendations, but I'll provide a quick idea: Buy a couple of index funds that cover the whole universe of investments. Index funds have low expenses and are the cheapest/easiest way to diversify. Buy a "total stock market" fund and a "total bond fund" in a ratio that you like. If you want, also buy an "international fund." If you want specific tickers and ratios, another forum would be better(or just ask your broker or 401(k) provider). The bogleheads forum is one that I respect where people are very happy to give and debate specific recommendations.
At the end of the day, responsibly managing your investment portfolio is not rocket science and shouldn't occupy a lot of time or worry. Just choose a few funds with low expenses that cover all the assets you are really interested in, put your money in them in a reasonable-ish ratio (no one knows that the best ratio is) and then forget about it.