What kind of spells and incantations do witches do?

That will depend on the witch. Personally I have cast spells for weather, for luck, for prosperity, for protection when I travel, and for health, just to name a few. Incantations are the Words (often in the form of a poem or rhyme) used to cast spells.

Not all practitioners use rhyming incantation, I tend to think it is unnecessary and makes getting the Wording "just right" some what more difficult. When casting spells one does not want any ambiguity in the Wording. The gods tend to be pretty literal in their response.