Koufuku Graffiti [20] stealthily, completely, well then etc

夜中（よなか）midnight, dead of night. Note it's not よるなか. Another word is 真夜中（まよなか）(from Persona 4 ^^;)

お見通し（おみとおし）this noun means 'seeing through someone's mind', 'correctly predicting their action' etc.

こっそり食べちゃおう
こっそり this adverb means stealthily, secretly

すっかり治ったよ！
すっかり this adverb means completely, thoroughly

よし いっちょ朝のランニング…
いっちょ（一丁）this has several meanings, but in this case means 'well then'

They're saying 鬼は外！福は内！('let demon be outside! let luck be inside!'), apparently when scattering beans (mamemaki) during Setsubun.