I suggest searching the web for "baby toy rentals" in the city where you are going. When my daughter was 7 months old, we took a trip to Phoenix and my husband found a rental place there. They rent age appropriate toys as well as strollers, etc. I was great since all the toys were new to my daughter.
Since that time, we have traveled a lot on airplanes with my two kids now 5 1/2 and 3 1/2. I highly suggest bringing your 3 1/2 year old son's car seat for the plane ride. He'll be higher so he can see out the window and he'll be in a familiar seat and won't be able to unbuckle it as well as the airplane seat belt. Since your 6 month old is a lap baby, you'll just need a few books which both kids will enjoy seeing as you read to them.
If you are flying Southwest, they are awesome in the "help" department. The captain once helped carry my carseats from the plane to the gate while I pushed the double stroller with the kids in it! Traveling alone with two kids is both fun and challenging. Don't forget to have fun!!!
My husband just sent me this websit used when we were in Phoenix.....http://www.thenewparentsguide.com/baby-equipment-rentals-...