What is "long term" to you? If you're looking at not breaking the tank down after a couple of years from a more active substrate (dirt, soil, ADA Aquasoil, Controsoil, Fluval Spectrum, etc.), I would recommend going with an inert substrate like sand, fine gravels, etc. (Yes your mileage will vary depending on substrate)
That would be different types of Sand, EcoComplete, CaribSea gravels, etc. Just choose your color. I wouldn't "cap" anything but active substrates (because you needed to keep it at the bottom of the tank), otherwise why would you cap it? By keeping it inert, you can use CO2 and liquid/solid ferts to keep plants healthy and aerate sand/gravel as needed without needing to break down the tank.
There's plenty of folks having success with their own method and recipe, so you can experiment if you want to do your own variation.