Board Thread:Geometry Dash Gameplay Discussion/@comment-29981603-20160916020136/@comment-29191679-20160916025306

Just a heads-up, you may know some of these.

Find some Easy-rated user-created levels with User Coins (the silver ones). Collect 10 to unlock the Vault, then type "Sparky". You'll get a free coin.

Scroll rapidly through the level selection screen to find a free coin in the Coming Soon page.

Beating Map Packs gives you secret coins.

Those are the easiest possible methods for obtaining secret coins.