If you use trees for walls, the succa is kosher.

A messenger who is doing a mitzva does not have to eat in a succa. A sick person and his attendant do not have to sit in a succa.

Casual eating and drinking can be done out of the succa.

Succa 2:5

There is a story with Reb Yochanan ben Zachi that they brought with him to taste some food and to Reb Gamlieil they brought two dates and a pail of water and they said bring it up to the succa.

When they gave to Reb Zadok less than an eggs worth of bread, he took it with a cloth and ate it out of the succa without benching.