Any method to earn lots of punya which is not required to be done regularly. It should be some sort of one time activity or anything which can be repeated again and again but there shouldn't be any obligation to do it or follow it regularly.
Krita Yuga - Dhyana. Treta Yuga - Yagna. Dvapara Yuga - Puja. Kali Yuga - Nama Sankirtan
कृते यद्ध्यायतो विष्णुं त्रेतायां यजतो मखै:
द्वापरे परिचर्यायां कलौ तद्धरिकीर्तनात्
Whatever result was obtained in Satya-yuga by meditating on Viṣṇu, in Tretā-yuga by performing sacrifices, and in Dvāpara-yuga by serving the Lord’s lotus feet can be obtained in Kali-yuga simply by chanting the Hare Kṛṣṇa mahā-mantra.
Vishnu Puran 6.2.17
dhyāyan kṛte yajan yajñais tretāyāṁ
dvāpare arcayan yad āpnoti tad āpnoti kalau saṅkīrtya keśavam
Whatever is obtained by dhyana in Satya-yuga, by yagnas in Tretā-yuga and by puja in Dvāpara-yuga, that is obtained in Kali Yuga by glorifying Keśava.