Hi there,
I had previously responded to some questions about this, so I copied it from my files... sorry if some things are duplicated from other responses!
We just finished a couple of planes trips with our 26 month old, one of which I did myself. You do need to buy a seat if they are 2. Some airlines have reduced children's fares, but often, if you buy far enough in advance, the regular fares are better.
I bought the CARES harness to use with my son, since I didn't want to bring the car seat on the plane. It's a harness (FAA approved) that you attach to the airplane's seat belt to create a 5 point harness for your child. I just felt it was a little safer than just the seat belt for him, since I was afraid he would wiggle out. They are a little pricey, but well worth it for me.
If you are meeting family on the other end, maybe see if they can get a car seat at a consignment store or someplace like that so that you don't have to bring one with you. If you do end up bringing it, though, most, if not all, of the airlines will check it for free so you won't have additional baggage charges. My sister bought a car seat at a yard sale, in good shape, for $15, and it worked out great for us.
Gate check the stroller. I went to the counter before boarding to get the gate check ticket, it made things a little faster.
We also got a "2 in 1 harness buddy" .. basically it's a little backpack (ours is a monkey) that you put on the child's back. It has a strap that you can hold onto and let your child walk. The strap can be removed. We didn't use it a lot, but it was very helpful at times when there were a lot of people around and he wanted to walk. It was especially helpful when going through security. That was the hardest part for me, because you have to fold up the stroller and put it on the belt, in addition to any carry on luggage. You can carry your child through, or they have to walk through on their own, While I was getting everything onto the belt, I was able to keep him close with the harness until I was ready to take it off. You child's shoes need to be removed in addition to your own, something to keep in mind.
Toys, snacks, DVD player all were good. And another little trick that I used... lollipops. I got little small ones that were safe for him (as long as I was right there watching). Because we don't give him candy, it was a novel thing for him, plus, it was helpful to get him to suck during takeoff and landing, and relieve the ear pressure. It worked with our son, because he just sucks on it, never bites. You would have to be careful if your child would bite it.
If you have particular food or drinks that you want your child to have, you can bring them along (even if they are more than 3 ounces). You just need to separate them out from you luggage so they can be screened. I did this because our son loves milk, and the planes don't usually have it. You could probably buy some of the things once you go through security, but I preferred to have it with me so there wasn't one more thing I needed to think about!
Whatever airline you chose probably has a section on their website with hints for traveling with children. You may want to check when you are booking your ticket.