The items, levels and characters all still work in giants from the adventure packs. The levels actually show up in your chapter select listing though, but are fully playable. I saw a bunch of them at Best Buy when Series 1 Giants launched, but to be honest I haven't seen them in stores since then.
So the items from the original game work in Giants? I'm not sure I've seen any around, lately, but right after Christmas, my son was looking to spend some of his Christmas money. Since he had just gotten Giants for Christmas, he was new to the Skylanders thing, and I had just heard about Slam Bam. I knew he would like him, and I ended up finding the adventure pack and the single figure at Target. I figured it would be better if he saved his money and just bought the single figure, but I didn't know what the items did or if they would work with Giants. He also keeps seeing things with the pirate ship online. I'm not sure if I could find it, but he wants it, but I wasn't sure if it would work or not. Although, I figure I might find a way to get him the original game sometime down the line, anyway, if I can find it cheap enough. I know I've seen it for $35 on Amazon (the full pack with portal and figures), but just not sure if I'm ready to pull the trigger, yet, financially, especially since I'm picking up so many figures for him for his birthday.