1. People who travel for mundane purposes are exempt from Sukah while they are traveling.
2. People who travel for the purpose of a Mitzvah are exempt from Sukah at all times.
3. If someone is sick, he and those who care for him are exempt from Sukah (while they are caring for him).
4. One is allowed to snack outside the Sukah, but he may not take a nap outside the Sukah.
5. The Gemara discusses what foods a person may eat outside of the Sukah.
A BIT MORE
1. However, if they are traveling only during the day and staying in a certain place at night, they are obligated in Sukah at night. They are exempt only while actually traveling.
2. They are exempt from the Mitzvah of Sukah even when they stop traveling at night.
3. If one finds it painful to be in the Sukah (for example, he is a weak person who cannot stay outside for a long time without difficulty), he is exempt from staying in the Sukah, but those who assist him are obligated to fulfill the Mitzvah of Sukah.
4. Rav Ashi says that this is decree mid'Rabanan, enacted out of concern that the person would sleep normally (and not just nap) outside the Sukah.
5. The Mishnah says that Rebbi Tzadok ate food (bread) that was less than the size of a k'Beitzah outside of the Sukah.