Commercial real estate in the form of office space is a liability. HOWEVER! Commercial spaces that are better for other things can be good. Industrial zoned real estate is generally a better bet these days IMO. Also, look around for mini-storage property. For whatever reason, people need a place to store their random crap, and the cost of upkeep and construction for these building types is relatively minimal. You need fencing and some form of security, but that's pretty much it. Another one along those lines is RV parking facilities. You can get by with covered parking (carports, but for RV's) and you just need a good bit of space, paving, and fencing/security. Huge returns relative to what it costs to build.
Caveats are that even industrial/warehouse space needs tenants, same with RV parking. So if the economy does take a massive dump as many are predicting, you can likely get some turnover in your tenants and finding new ones may be a challenge. Vacancies will kill you if you're doing all this stuff via bank loans.