Of the forty objects meditated upon as kammatthana, the first ten are ‘things that one can behold directly’, ‘kasina’, or ‘a whole’:

(1) earth, (2) water, (3) fire, (4) air, wind, (5) blue, (6) yellow, (7) red, (8) white, (9) enclosed space, (10) bright light.
The next ten are objects of repulsion (asubha):

(1) swollen corpse, (2) discolored, bluish, corpse, (3) festering corpse, (4) fissured corpse, (5) gnawed corpse, (6,7) dismembered, or hacked and scattered, corpse, (8) bleeding corpse, (9) worm-eaten corpse, (10) skeleton.
Ten are recollections (anussati):

First three recollections are of the virtues of the Three Jewels:
(1) Buddha
(2) Dharma
(3) Sangha

Next three are recollections of the virtues of:
(4) morality (Śīla)
(5) liberality (cāga)
(6) the wholesome attributes of Devas

Recollections of:
(7) the body (kāya)
(8) death (see Upajjhatthana Sutta)
(9) the breath (prāna) or breathing (ānāpāna)
(10) peace (see Nibbana).

Four are stations of Brahma (Brahma-vihara):

(1) unconditional kindness and goodwill (mettā)
(2) compassion (karuna)
(3) sympathetic joy over another’s success (mudita)
(4) evenmindedness, equanimity (upekkha)

Four are formless states (four arūpajhānas):

(1) infinite space
(2) infinite consciousness
(3) infinite nothingness
(4) neither perception nor non-perception.

One is of perception of disgust of food (aharepatikulasanna).

The last is analysis of the four elements (catudhatuvavatthana): earth (pathavi), water (apo), fire (tejo), air (vayo).

Each kammatthana can be suggested, especially by a spiritual friend (kalyāṇa-mitta), to a certain individual student at some specific point, by assessing what would be best for that student’s temperament and the present state of his or her mind. source: wikipedia