I don't have the Collector's Edition, but I think the "extras" include the following:
a nice map
two bonus merchants added to the game who sell items from Icewind Dale and Planescape: Torment
As far as I know, that's it. You can add the bonus merchants to the game yourself by downloading a couple of small files. I don't know how much the map is worth, but you might like it.
If I'm wrong about the Collector's Edition, I'm sure that someone here will correct me.
If you want to add new quests or new characters to the game, you can download all sorts of unofficial "mods". There are a lot of mods out there that add new quests to the game. The best one I can think of is the Ascension MOD for Throne of Bhaal. It was created by one of the original BG2 designers (although he goes to great pains to inform everyone that his mod is not "official"). It changes the ending of the game dramatically. If you can beat the new monsters (it's very difficult), you might like it.
There are also mods that add new characters to the game. Most NPC mods add new dialogues to the game and a small quest or two for the new NPC, but some mods add a lot more than others. The best NPC mod I can think of for Shadows of Amn is Keto, but the Keto MOD for Throne of Bhaal isn't finished yet. (Even though the game is several years old, the modding community is alive and well.) There are also various mods that add new items to the game, as well as mods that fix bugs or change the rules of the game to make it more convenient and more enjoyable. For example, I always install a mod called Ease-of-Use to remove the stacking limits--that is, instead of only being able to put 40 arrows into one stack, you can put as many as 65,535 in one stack. You can also add new kits to the game or change the abilities of the existing classes. For example, the Rogue Rebalancing MOD seems to be fairly popular, and I think the Shapeshifter Rebalancing tweak makes a lot of sense.
I'd rather not go any further unless you're interested in unofficial mods. If you are, then the people here can give you a lot of recommendations.