Aside from equities & bonds, what other asset classes are available for a retail investor?

There are 6 asset classes available.

Equity
Bond
Property
Commodity/Collectible
Cash
Alternative/Speculative

For commodities, you can learn to trade futures on their prices, hold them physically like gold, or buy funds that invest in such commodities. This would also include things like luxury watches, wine, art.

Lastly, for alternative or speculative investments, you could look at forex, crypto currency, some form of short term trading, even gambling.

I'd say you can try them out and develop an interest, but I'd still probably limit to 5% of my portfolio for each of these 2 asset classes.