My DS is 3.5 and weighs 33 lbs, we will be traveling in December by plane and would like to know what is the best option when it comes to a car seat. Renting a vehicle may be a possibility once we reach our destination, although not sure yet. What do you all recommend?

My 32 weeker is now 3.5!
Re: Carseat and traveling
ditto this.
We're taking a cruise in February - DD will be over 4, but still around 35 lbs. Since we won't be in cars much, I don't want to bring the full carseat, but I'm going to get a portable booster to bring, but I won't use it on the plane. She's flown 2x w/o her carseat and she's much happier in a regular seat.
This. With all the crap you have to lug around, do you really want to drag a carseat as well? No thank you.
We gate check our car seat and use the GoGo Babyz to lug it around and that works very well (we travel a lot is it was worth the cost). We used the car seat on the plane one time and will not do that again. Most of the time we fly Canada regional jets and they are little. Pain to take the car seat on and no room in the seat and lots of kicking.
since DD turned 2 we have been using the Cares harness and like that a lot. She stays in it well and knows she cannot get out until the seat belt sign is off. Then we let her out and she can stretch out on her seat. The one time we flew with a car seat on the plane she napped in there and then was on my lap anyway (when she was 16 months). that was an 8 hour flight so there is no way she would've stayed in her seat that whole time.
We always have a rental car at our destination. We went to New Orleans when she was 9 months when we did not need a car. She was still in her bucket seat at that point but it wouldn't fit in the shuttle so she sat on my lap anyway.
We will likely travel to Rome in March and won't be bringing it at all (other than the ride from our house to the airport). She will sit on our lap for the cab ride from/to the airport and the rest we will walk.