What should be avoided while fasting

1. Continuous fasting.

Abû Hurayrah said,
‘the Messenger of Allâh (sallAllâhu ‘alayhi wa sallam) prohibited continuous fasting whereupon one of the Muslims asked him, “but you do it O Messenger of Allâh!” He replied, “who amongst you is like me? I spend the night with my Lord, He giving me food and drink.” But when the people persisted in fasting continuously, he fasted continuously with them for a day, followed by another until they saw the new moon upon which he said, “if the moon had delayed in appearing I would have made you fast more.” This was a punishment for them because they refused to stop continuously fasting.’
[Bukhârî and Muslim]

Continuous fasting was prohibited because it weakens the person and emaciates his body by reason of something other than worship. As for the Messenger of Allâh (sallAllâhu ‘alayhi wa sallam) then if it is literally true that his food and drink was with his Lord, he cannot be said to have fasted continuously; but if ‘food and drink’ is a phrase used to denote the strength of his closeness and communion ( uns) with his Lord and the joy he felt at this closeness, then this takes the place of food and drink in revitalizing his strength, indeed this uns is better than food and drink in this respect,

Indeed I fasted from worldly pleasures my entire life
And the Day that I meet you will be its breaking

For surely I find the delight of fasting for You within my soul
And not in the satisfaction of eating or drinking

2. Kissing

‘آ’ishah (radiyAllâhu ‘anhâ) said,
‘The Messenger of Allâh (sallAllâhu ‘alayhi wa sallam) would kiss and embrace while fasting but he was more able than you in restraining his desires.’
[Bukhârî and Muslim]

Therefore whoever is old, able to control his desires and hence not ruin his fast, there is no harm in his kissing. If one is young and not sure that he can control his desires, it is disliked for him to kiss due to his subjecting his action of worship to the danger of something that could break it.

3. Cupping

It is authentically reported that the Messenger of Allâh (sallAllâhu ‘alayhi wa sallam) was cupped while fasting. [Bukhârî] Anas was asked,

‘did you consider cupping to be disliked for the one fasting?’ He replied, ‘no, [we only considered it disliked] if it caused weakness.’
[Bukhârî]

Hence whoever is physically weakened by cupping, it is disliked for him to do so, because in his weakened state he is more likely to break fast or find the action of fasting extremely difficult for him and hence become averse to it.

4. Applying Kohl.

Anas would apply Kohl while fasting.
[Abû Dâwûd and ibn Hajr said the isnâd had no fault.]
al-A’mash said,

‘I never saw any of our companions disliking the application of Kohl for the one fasting.’ Ibrâhîm [an-Nakha'î] would allow the one fasting to apply aloes to his eyes.
[Abû Dâwûd]

Having said this, it is safer not to apply it so as to steer clear of the difference that the scholars have concerning this issue.

5. Snuffing the nose during ablution

The Messenger of Allâh (sallAllâhu ‘alayhi wa sallam) said to Laqît bin Sabrah,

“perform the ablution well, wipe between the fingers and exert yourself in snuffing the nose, unless you are fasting.”
[Abû Dâwûd]

So he prohibited from exertion when snuffing the nose while fasting, the reason for this is that through exertion one puts his fast to risk [because the water could reach the back of the throat].

Allâh knows best.

[Actions which nullify the fast are eating and drinking deliberately; making oneself vomit; menstruation and post-natal bleeding and sexual intercourse.]