General:
* Souvenir
* Memento
* Token
* Treasure
* Relic
* Curio
* Heirloom (if passed down through generations)
More specific:
* For a loved one: Trifle, trinket, token of affection
* For a special event: Commemorative item, memento mori
* From a trip: Travel souvenir, travel memento
The best word to use will depend on the specific item and the reason it is being kept.