People are always looking for new and innovative ways to invest. I am not going to portray myself as being an expert on any of these but do feel there is value in being made aware that they exist and providing you all with the concept around why some invest in them. As with any investment, I think it bears mentioning that good general advice is to invest in what you know.
The key benefit I see to many alternative investments would be that they tend to have a low correlation to the traditional markets, meaning they can possibly see high returns when the stock or bond market is down. It means you can be more diversified and potentially see smaller swings in the value of your portfolio. That can be a benefit when the stock markets are down but could also prove to be a limitation on your gains when markets are moving higher.
There are plenty of educational resources out there giving you detail on how this all works. I would be mindful that there are always people out there looking to just make money off you so be careful of the "get rich quick" type schemes.
Real estate has historically been a consistent performer and can be a valuable piece to a strategy. Physically owning real estate requires vetting renters, dealing with repairs and upkeep, property taxes, and a multitude of other tasks. You can avoid much of this by investing into Real Estate Investment Trust, also knows as REITs.
When you invest in a REIT you are investing into a company that specializes in owning, operating, or financing real estate. One benefit to be gained by investing in REITs compared to buying real estate directly is diversification. Like a mutual fund that diversifies by investing in several different companies a REIT operator often owns and manages a mix of property types which can be located over a wide geographical area. REITs usually grow in value when the rents they collect on their properties goes up and when the underlying property they own and operate appreciate in value.
The common precious metals used in investments are gold, silver, and platinum. We tend to see lots of commercials for previous metals, especially during market downturns and particularly during times characterized by having higher inflation. This is because they tend to have higher returns during these times as they sometimes have an inverse return to stocks (precious metals go up when stocks go down and vice versa). What they may not tell you is that historically stocks (like the S&P 500 index) have typically seen better returns over the long term. Historical returns are no guarantee for future returns but it is a data point to consider.
Another downside to previous metals is that they are not income producing, meaning they derive all of their value by increasing in price as opposed to stocks and bonds which can have income from dividends and interest to produce value consistently. They are a legitimate investment and can be used as a piece of a properly diversified portfolio.