Hi A.,
Someone asked just this question yesterday, in regard to her son and new daughter. I'll post the link, and paste in my answer from that:
http://www.mamapedia.com/questions/1243269263196815361
Kids like to play where the action is.
In your case, here are two suggestions that give you control and some safety over the situation.
First, when your bigger one wants to play with "little pieces" but be close to you, set up a portacrib/pack-n-play in the livingroom. That is HIS space to play, and the little pieces must stay in the portacrib. This way, the pieces are contained (he can come back to them) and might not fall on the floor, much as they could at the table. (Which is another option, working "up".)
Also, consider investing in a good gate for your son's room. This way, when your daughter is mobile, you can gate the room off. He can play in there but still feel more connected because the door won't be closed. This will also keep out little sister. I like the gates which have hinges and lock into place instead of the pressure/tension gates which can fall over.
*added: having a separate play area before baby is mobile also helps, because older kids can get pretty elaborate in their imaginative play, setting up 'stories', so having a gate they can play behind, or a table up higher, lets their work remain undisturbed, which is one sort of sibling tension when baby becomes more of a person and mobile.
As time goes on, you'll discover that you invent other ways to keep your older child's play protected and separate. I would also be clear that if little pieces are left out, that they go away for a week or so. You can make the clean-up fun, though, and be :"hunters" for whatever you need to pick up. Make a game out of it.
(When I was 8 and my brother very new, we had a rule when sewing that she would count out all of our pins... if any one was missing when we cleaned up, we'd have to search until we found it, so our little brother wouldn't choke. Just an idea for seriously dangerous things for baby: counting out how many and then making sure all of them are cleaned up.)