Agree with the other posts about toy ideas... However, the first time we brought our toddler's car seat (Britax Roundabout) on the plane, we did not know the tray would not open for her because of the way the car seat rests in the plane seat. Britax's have a rounded front that bumped the tray up too much. That made it pretty much impossible for her to play with the play-doh kit she was WAY excited about. We somehow made it work. You didn't say if you're bringing hers, but I thought it was worth mentioning. (we learned it's a comfortable/safe place for our toddler and when she's strapped in, she's much MORE content)
Little dolls were a big hit for my older daughter on one flight. I think they're by Mattel, but not "sexy" like Barbie. They have cute outfits, normal bodies, and lots of hair to play with.
Doctor kit (an X-mas gift from grandma) was a HUGE hit. Just have to be careful to not lose the pieces. I like the idea of band-aids.
If you have an iPod that supports video (i know you said no laptop or mini DVD player), you can download a few episodes of her favorite show or a kids' movie, inexpensively usually. Just make sure you have earphones the 2.5 year old can wear. We found on evening flights, that's the best activity for our daughter. During daytime flights, she really loves to just play. We're now moving away from toys and transitioning to magazines/activity books/picture books with plots and our favorite--making up stories! She's flown a bunch since birth, so she's used to it now (same age as yours), which mkes it easier.
Another thng we've done is purchased a cheap little backpack for our toddler. Then we let her "pick" out a few of her own toys to bring along. This is especially helpful at the security gate, because it gives her something to be "responsible" for and helps her feel like a big kid. I'd also guess that it helps her feel like she's had some control over the traveling, which is a harried experience.
Most of all, hype up how "much fun" it's going to be! Tell the kids you're going on "an adventure" and take lots of pics!