I'll list as many as I can.
- Bottle of spring water -- Jesse gives Winnie the bottle of spring water that if she drinks it, she will live forever. That's a big temptation for Winnie. She ends up giving it to the frog though.
- Treegap Jail -- not really an item, but it is important because Mae was put in it after assaulting someone. That's a big deal, because Mae is portrayed as exceptionally kind.
- The woods themselves -- This is where the Tucks live and it represents safety for Winnie and freedom from her hovering parents. It's also full of mystery and magic.
- The spring itself -- the source of the water that grants immortality
- The toad -- In Shakespeare plays, a character would just turn to the side and say his aside. It was a device used by playwrights that allowed characters to express their inner thoughts. In "Tuck Everlasting" the toad allows Winnie someone to talk to about her thoughts. It's a little weird to talk to a frog like that, but less weird than just talking to yourself I guess.
- The music box -- It belongs to Mae, and she never goes anywhere without it. While most people have never seen the Tucks, there are stories of "elf" music coming from the forest. The box and it's music are evidence that there is "something" in those woods.
- The trout -- Miles catches the fish, and it starts to die in the bottom of the boat. Winnie begs Miles to put it back, so that it doesn't die. Perhaps the scene is hinting at what Winnie will decide in the future -- everlasting life or death?
- The pond -- Tuck tells Winnie that the pond is a symbol of constantly changing life.
- The boat -- Tuck tells Winnie that the boat represents his family. They are on/in the changing river/pond of life, but stuck in one location. Like a boat tied to a dock. Life changes all around them, but they do not.
- The Tuck's home -- again, more of a location than an item, but it is the place where Winnie feels most at home. The Tucks treat her as a member of the family and treat her with respect. They give her responsibility.
- Winnie's home -- The place where Winnie feels confined and constrained by her hovering and doting parents. They don't give her much of any kind of freedom, which is perhaps why Winnie is so drawn to the Tuck and their home.