There are virtually no high return investments that are both passive and safe. If you are looking for solid returns that are relatively safe (i.e. low beta) you will need to look at non-traditional investments. Many non-traditionals are able to return better for two reasons.
First, they are often not passive, meaning it requires incremental effort apart from their purchase. Second they exist in inefficient markets. This simply means you will have to work to find the opportunity rather than simply purchasing the investment via an exchange.
Take a look at things like tax sale properties, personally held mortgage notes, or antiques. Each of these require significant homework before you enter the space, but they all exist in inefficient markets, which should provide for better returns and you can often determine the value before the purchase, thus locking in reasonable returns at the time of purchase.