Your question seems to have two parts, but they are in fact related. The object as motif is a frequent conceit in this film, and there are certain themes underscored by the repeated appearance of certain objects. As well as danger and the supernatural, one frequent theme is the unknown, as well as clairvoyance (visions of the future). Loss (of people) and being lost are also themes that repeat throughout. The theme of repairing or mending the past functions on several levels: John and Laura move to Venice to relocate from the site of their tragedy; John literally repairs the past when he restores the chapel mosaics, and Laura attempts to repair the past when she seeks comfort from the blind psychic regarding the death of Christine.
As for objects that act as motifs, these include broken glass, the color red (seen in blood, John's mosaic work, Christine's rain jacket, and the coat worn by the murdered in the end of the film), rain and water, and running (as when John;s son runs to tell him about Christine, and John runs to save her, and he runs when he and Laura are separated in the streets of Venice.
The whole film is an elaborate association of motifs. Don't Look Now is such a fascinating film to study, I think, since so much thought went into its construction.
One of the main themes is separation. In the film lots of things that are whole separate into parts. The mosaic for instance that John is restoring is a whole constructed out of parts, and there is a preoccupation with breaking glass. The Baxters lose their daughter, and become separated by events in Venice, and even during the scene in which we see John and Laura make love, the film keeps cutting to them getting dressed separately. So we see them together, and then apart and back and forth - it reinforces the feeling that even though they are together there is separation between them. Separation isn't just a theme of Don't Look Now, it occurs in all of Nicolas Roeg films, perhaps the most prominent examples being Walkabout and The Man Who Fell to Earth.
Another theme that is central is misinterpretation. At one point John says to Laura "Seeing is believing...I believe you". It's such a fantastic line in the context of the film. We put so much stock in observable evidence - science is built on the principle of deriving theory to explain observable evidence. Don't Look Now takes the stance that interpretation informs observation, so it's just as fallible as all other beliefs. John has a vision of Laura on the barge but mistakenly believes she is presently in Venice, and sets in motion the chain of events that ultimately cost him his life. He does it again near the end when he mistakes the red-hooded figure for a child. This happened to Daphne du Maurier in real life; she mistook a dwarf for a child and it caused her considerable embarrassment. But it gave her the premise for the story. Misidentification is extended thematically in the film. When John is describing what the two sisters look like to the detective, he looks out of the window and sees them but doesn't make the connection between what he is looking and what is being described. Later on when the detective meets Laura he tells her she doesn't look much like her photo.
An important thematic motif is the association of danger with falling. Christine drowns by falling into the lake, and Laura is hospitalised after her fall in the restaurant. She later returns to England after their son has a fall at school. John of course nearly dies in a fall later in the film, and shortly afterwards the bishop confides in him that his father died in a fall.
Water is constantly associated with death throughout the film, starting with Christine's drowing through to the murder victims being fished out of the canals, to the very idea of Venice itself being submerged, a city in the process of drowning.
Perhaps the most famous motif is the foetal shape of Christine's body as John pulls her out of the water. When the water spills on the photograph of the church it leaves a foetal mark on it. There is a foetal shape in the mosaic John is restoring, and one of the sisters has a foetal shaped brooch on her coat lapel. This shape is tied to the image of their deceased daughter at the start of the film, and is a recurring image that is a subliminal constant reminder of the dead child. It is clever techniques like this that give the film its pervading sense of loss.
I hope you find some of my suggestions useful.
Suggest reading the Daphne du MaURIER 1971 short story " Don't look now" which with minor changes forms the wholestory line of the film.