I save all my idle coins and let them grow safely in real-world investment porfolios (that are denominated in BTC).

This is not high-yield investment programmes (HYIPs), as I think those are too risky.

Invest globally with bitcoin. Wealthfront for bitcoin.
http://themerkle.com/bitcoin-investment-startup-keza-unveils-ios-beta-app/
https://www.producthunt.com/tech/keza
http://getkeza.com/ (It's free to apply for a Keza account)