Is it permissible to perform a ritual bath without a reason, i.e., in the absence of major ritual impurity (janabah), menstruation (hayd), post-childbirth bleeding (nifas), or a Friday, or is it impermissible and considered an innovation (bid'ah)?

There is no harm in taking a ghusl (ritual bath) with the intention of cleanliness or cooling off. This ghusl would be similar to the manner of the prescribed ritual baths performed as acts of worship. However, performing ghusl as an act of worship with the intention of removing janabah (ritual impurity) when one is not in a state of janabah, or performing ghusl for Jumu'ah (Friday prayer) before its day, is not prescribed and is an innovation (bid'ah). This is based on the Prophet's (peace and blessings be upon him) saying: "Whoever performs a deed that is not in accordance with our matter, it will be rejected," and his saying: "Indeed, actions are by intentions, and every person will have what he intended."