Placing some amulets of power in some hidden spots or as quest rewards could be a solution - at least one for each character.
Experience is a part of character progression. It's a reward for questing and combat, the same way you get loot for defeating enemies or finding chests.
Some people don't mind, but personally, when I hit a level cap, I lose interest in continuing very quickly. All the things that you're doing that would normally provide experience no longer have any tangible effect (except for boss loot drops and some minor gold rewards). It begins to feel like you're just spinning tires when the characters stop improving.
I don't really care about skill points. They're nice, but the experience reward itself is an important part of the system, and there's no cheap way around that. I wouldn't want the final piece of post-game content to come out with really only the story elements and possible loot (that isn't going to have much use, since it's all taking place after the end of the main game) as the only sense of progression.