I am having the same problem. My son will be 2 in a couple of months and I have a 4 month old daughter. We have regular toys, learning toys, and special toys. All are kept in different places. We put our son in a mother's day out program 2 days a week this year. He loves it and we are trying to keep the same schedule at home. 15-20 intervals with activities. He can play with regular toys any time, the learning toys are different activites we do with him where there is more one on one interaction with him while the baby is sleeping, and the special toys are pulled out when regular toys are put away. We taught him to put his toys away before lunch, before special toys, and before bath time. We made it really fun for him. With this way we have notice that he does not get as bored with more consistency and teaching him responsibility. But, a couple of weeks ago we bought him so fish. 2 male bettas and a split tank they make for bettas that came with everything. He LOVES the fish and feeds them in the morning when he feeds the dogs and in the evening when he feeds the dogs. We put them on his little table in the kitchen and he will sit there and watch them for about 45 min in the morning and 45 min in the afternoon. It has not gooten old yet, and for less than $15 at Wal Mart I have a little bit more sanity during the day than I used to.
We also work with him on helping us with little things and I think that makes him feel needed and as special now that there is a little girl that occupies Mommy's time as well. We always praise him and clap our hands when he helps us. He gets so excited to help for a few minutes that it is easier for him to then go back to his toys for another 30 min and forget he got bored with them earlier.
Movies are also a special treat. We let him watch baby einstein videos and since we had him listening to classical music while I was pregnant and in the first couple of months, he thinks it is the coolest thing to sit with his baby sister and watch baby bach or baby beetoven. This has taught him that his sister is here to stay and he can do things with her like Mommy and Daddy do.
Good Luck