"If you invest your money now, you might be able to make something like 10% annually with some risk."

10% is rather questionable as a long-term prediction: only the most successful equities markets in the world managed close to that in the last century, and many disappeared entirely. After corrections for inflation, survivorship, taxes, and so forth real returns are closer to world growth rates.

Speaking of this, does anyone know of any LW posts or other articles about how to make the most of idle capital with some risk?

The ultra-standard advice of economists is index funds. They beat the overwhelming majority of actively managed funds in any given year and outperform active management (even the rare few hedge and private equity funds that seem to have real repeatable skill just raise fees until the marginal investor is indifferent) via low fees and just holding the diversified market. Basically, it's very hard to beat the market, and management and trading fees are costly, so index funds don't try to do the former and clean up on the latter.

Anna and I invest with Vanguard, which we picked based on low fees, large size, and good reputation, but almost any brokerage will offer them. You can just go to one of their websites with your bank information at hand and get things set up quite rapidly (it took less than an hour for us to invest at the Vanguard website in global equity index funds, seeking high diversification and exposure to the historical equity premium).

You can search LW for "index fund(s)" if you want, and find some economists (James Miller and maybe Robin Hanson) among others saying this, but I would go for index funds based on the track record and the overwhelming consensus of financial economists rather than LW opinion.