There are unlimited choices, actually -- unlimited varieties of service to Krsna that one may be inspired to perform in a heartfelt, sincere, prayerful and humble manner to earn the wonderful benefits of karma-clearing and spiritual health for oneself and others. In the age we currently live in, called Kali-yuga, the most emphatically recommended seva, or service, is that of lovingly reciting, chanting, loudly singing or calling the transcendental and supremely purifying holy names of the Lord. There are unlimited such names, which may be called individually or uttered in the form of different standard songs and mantras, but what is called the maha-mantra, or the Great Chant for Deliverance, is our famous Hare Krishna chant --
Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare/
Hare Rama, Hare Rama, Rama Rama, Hare Hare.
(pronunciation: "Hare" -- huh-ray; "Krsna" -- Krishna; "Rama" -- rhymes with drama.)
Since the Lord is nondifferent from His names, hearing and/or chanting these names means literally being in His personal and unlimitedly purifying presence, so the more these names are heard and sung, the better for everyone involved. Thus we try to chant, for our own benefit and that of everyone else, as much as we can. And there are some devotees who consider some type of chanting of Krsna's names to be their favorite service or their calling in life -- whether it be public harinama sankirtana (singing the holy names out loud together with others in public places), temple or home kirtanas (doing the same but in the temple, at home, or in some other more private setting), making recordings of the singing of the holy names and offering them for sale, or simply chanting japa (an intensely prayerful session with Sri Sri Radha-Krsna in which a fixed minimum number of Their holy names [in the form of the maha-mantra] are chanted on beads daily). The more loudly one chants, the more souls can hear it and be benefited, but all of these are bona fide ways of serving the holy names of the Lord.
Other awesome services that devotees may be inspired to perform include:
- Sharing the knowledge that has transformed their own lives with others by either distributing books and articles written by other devotees (and/or sharing and explaining the realizations they've gotten through reading those works), or writing their own.
- Using their creative talents in Krsna's service by drawing, painting, sewing, carving, or sculpting images of Him and His devotees (which will thereafter enlighten, inform and purify the minds of anyone who views them by teaching those souls about -- or reminding them of -- Sri Krsna or His wonderful servants), or by creating beautiful clothing and accessories for His arca-murti (the forms of the Deities worshiped in the temples).
- Practicing the love-laden worship of the personal form of the Lord by offering those Deities all kinds of auspicious, nice and desirable things.
- Purifying themselves and others by partaking of, and enabling others to partake of, the remnants of these offerings to the Deity -- called prasadam, or mercy, for their amazing spiritually transformative effects -- in the shape of delicious foods, fragrant flowers, sanctified water, etc. etc. (The distribution of prasadam food enjoys particular popularity due to its ability to nourish those who consume it both spiritually and materially at the same time.)
- Orchestrating, assisting in the production of, or performing at festivals, dramas, dances, and other cultural exhibits, which serve the purpose of charming and capturing the minds and imaginations of those who attend, wrapping their consciousness in Krsna and helping them make progress toward Him.
- Keeping the Lord's temples (including their own homes and, for that matter, their bodies) sparkling clean and functioning as they should, so that the Lord's service can be carried on appropriately therein.
- Offering their expertise in any field of service to Sri Krsna and the devotee community -- e.g. teaching, counseling, legal services, medical care, cooking, gardening or farming, animal husbandry, managing / administration, website development and maintenance, etc. -- either for free or in exchange for necessary income.
As you can see, the options are truly unlimited. Either as inspired by Paramatma / caitya-guru (Krsna in one's heart, a.k.a. one's conscience), or as advised by a trusted spiritual guide (guru) with whom it is possible to converse face-to-face, or by a combination of both (possibly along with other factors), each individual devotee can arrive at a conclusion regarding what his or her daily services ought to be, and then proceed to execute them with as much energy, sincerity, hope, humility, determination and enthusiasm as can possibly be mustered. This process is known as bhakti-yoga, and when executed properly (especially under the caring guidance of a trustworthy personal guru who has made more progress than oneself in successfully achieving a life of devotion to Krsna, who will help make sure one stays on the right path and doesn't fall prey to any of the traps and temptations along the way), it leads in short order to the sort of genuine, overflowing, radiant happiness that will make the devotee's own life successful and be magnetically attractive to the suffering people of the world, who may even approach the devotee to find out what his or her secret is.
In order to be really effective at helping others, one first of all must help oneself. It's important to make sure that in one's zeal to help other suffering souls drowning in the material ocean, one doesn't capsize one's own spiritual boat in leaning out to help them in. That will help no one. First of all make sure you are executing spiritual life properly -- i.e. make sure it's working for you, yielding tangible results in the form of deep joy, peace, satisfaction, valuable realizations and character development -- and make sure your spiritual health and progress are stable, reliable, unshakable, and steady rather than the result of unsustainably, fanatically pushing yourself to achieve levels of perfection that are currently beyond your ability to maintain long-term. Only then will you be able to provide help and advice to others that's effective, powerful and valuable.
Some devotees actually belong to a category called bhajananandi, which means their focus is primarily or exclusively on private worship of the Lord and on making spiritual progress themselves. The alternative category of devotee is called gosthyanandi; these devotees feel joy when they see other souls finding success and happiness by coming to Krsna consciousness, so they endeavor to help that happen as much as possible. Both types of devotees are glorious and dear to the Lord, but the latter category of devotee is more so. As Srila Prabhupada explains in his wonderful purport to Srimad-Bhagavatam 7.6.24, Sri Krsna states in Bhagavad-gita 18.69 that "there is no servant in this world more dear to Me than he, nor will there ever be one more dear" than one who engages in preaching Krsna consciousness. This confirms that there is no method of doing good in the world that is greater than this.
May we all engage in it according to our respective realizations, inspirations and capacities and thus please Sri Krsna, the Lord of all, and make our lives successful and ecstatic.
All glory to Sri Sri Gaura-Nitai's mission of maximum mercy, and to all the Vaisnava devotees of the Lord!