All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The Muslim scholars have different opinions concerning the matter of covering the face and palms in front of marriageable men if there is no fear of temptation in uncovering them. There is no disagreement among them that covering them both becomes obligatory if there is fear of temptation in uncovering them.
As for feet, according to the majority of Muslim scholars they must be covered by a free and pubescent woman in the presence of marriageable men, for the narration that the Prophet said: "On the Day of Judgment Allaah will not look at the person who drags his garment with pride and arrogance", so Um Salamah inquired: "What should women do with the lower end of their garment?" The Prophet replied: "They should extend it the length of a span of a hand." She said: "Their feet will thus be exposed?" Thereupon, the Prophet said: "So they should extend it the length of a cubit and should not extend it further than this." [At-Tirmithi and An-Nasaa'ee]
Therefore, wearing sandals is allowed if her garment covers her feet. If the garment does not cover the feet, then she should wear thick socks in order to cover her feet.
Allaah Knows best.