The deceased's personal belongings such as clothes, watches and jewellery — must they be divided by faraid, or can they be given away in charity at once?

All of them are part of the estate belonging to the heirs; they may only be given away after all adult heirs agree, and the share of heirs who are minors may not be given away.
Allah says heirs have a share of what is left, little or much (an-Nisa' 4:7) — including small items. The custom of giving away the deceased's clothes soon after the death is well-meant, but if there are young orphans, their rights must be protected, since consuming orphans' property unjustly carries a stern warning (4:10). Valuable jewellery, watches and phones are usually included in the list of assets. A simple approach: list the items, give a rough value, and let the adult heirs agree in writing on what is given away or kept as a memento. See Managing an estate.