I compared investing in real estate a few years ago to investing in stocks that paid double digit dividends (hard to find, however, managing and maintaining real estate is just as hard). After discussing with many in the real estate world, I counted the average and learned that most averaged about 6 - 8% on real estate after taxes. This does not include anything else like Dilip mentions (maintenance, insurance, etc). For those who want to avoid that route, you can buy some companies that invest in real estate or REIT funds like Dilip mentions. However, they are also susceptible to the problems mentioned above this.
In terms of other investment opportunities like stocks or funds, think about businesses that will always be around and will always be needed. We won't outgrow our need for real estate, but we won't outgrow our need for food or tangible goods either. You can diversify into these companies along with real estate or buy a general mutual fund.
Finally, one of your best investments is your career field - software. Do some extra work on the side and see if you can get an adviser position at a start-up (it's actually not that hard and it will help you build your skill set) or create a site which generates passive revenue (again, not that hard). One software engineer told me a few years ago that the stock market is a relic of the past and the new passive income would be generated by businesses that had tools which did all the work through automation (think of a smart phone application that you build once, yet continues to generate revenue). This was right before the crash, and after it, everyone talked about another "lost decade." While it does require extra work initially, like all things software related, you'll be discovering tools in programming that you can use again and again in other applications - meaning your first one may be the most difficult.
All it takes in this case is one really good idea ...