The “there’s thousands of user-created adventures!” aspect of NWN can be both a good thing and a bad thing. For someone just starting out, having to deal with importing/exporting characters between modules and then wading through all the crap to find the decent content can be a little overwhelming.
The vault does a good job of that though (nwvault.ign.com). Just go to NWN modules and go to the hall of fame. Everything there is gold.
I don’t think I’d introduce her to Diablo 2 unless you’re planning on playing tcp/ip or in password protected public games. Too many juveniles there.
Haven’t played Dungeon Seige 2, but I did play the first one multi a little bit. It’s okay, but kind of boring. The secret chicken level is probably the best thing about it, and if you’re capable of beating that (it’s very very hard), there isn’t much more you can do.
My suggestion would be Anarchy Online. You can play the basic game for free, the expansion packs require subscription fees. The base game is still very very large, with tons of stuff to do. You can level and have fun right off the bat, but there is a ton of little things (crafting, twinking, special items, etc) that will take you quite a while to figure out.
Having played AO myself (using the fr00b program), I would have a hard time recommending it to anyone.
In retrospect, it is insanely complicated and very difficult to learn the mechanics of the game (and there are many) from the ground-up and establish yourself financially. Planning out skill-point distribution and gathering materials for twinking could very easily become a full-time job (putting together a complete set of ql125 implants, or blitzing a full suit of armor, calculating how much treatment you need to install those new implants and factoring out stat loss when you remove your old set, etc, etc, ad nauseum).
Offsetting this is a very shallow gameplay wherein you are expected to spend your time hunting boss-mobs (which are badly overfarmed), hunting in a bare handful of static dungeons (which are horribly overcrowded), running randomly-generated missions (which are incredibly repetitive), or hunting mobs in the wilderness for little gain.
Plus, compared to other games out there right now, it’s pretty ugly.