Index fund is the way to go for many people. In fact, it is the suggestion William Sharpe gave to (pre-IPO) Google employees.
I would also consider buying Berkshire Hathaway stock. Expensive, B-series at about 4K, but worth it, again, long term. Slightly better returns too.
Even better, do not listen to me (IANA financier), but read Phill Greenspan's excellent investing advice [2].
[1] https://personal.vanguard.com/us/FundsIndexOnly [2] http://philip.greenspun.com/materialism/money