Food I regularly take on to a flight:
* Sandwiches
* Huge bags of crisps, or pretzels or tiny crackers or etc
* Chunkily sliced cucumbers, peppers, carrots
* Non-liquid dips like cream cheese or hummus (with the slight potential of having them confiscated, but so far so good)
* Dried fruit and nut mixes
* Fresh fruit
* Cherry tomatoes
* If I'm feeling posh, little supermarket deli things like stuffed pastries or cooked meat
I got used to it from budget airlines where there's no food provided, and now I do it even when there is - I think the airline staff and fellow passengers got used to it from the same influences. I find even when I'm sick of podcasts and books just having something tasty to munch while I listen to music etc keeps me happy. I always bring a bit more than I know I can eat so that I can have variety (helps with the boredom thing), and then it's useful emergency snackage at the destination too, so long as you don't go overboard on the fresh perishables.
It also helps to know what's in the airport. E.g. Luton has an M&S before security (good for all of the above, if you want to buy them on the day), and once you're through, there's a Wasabi (which has lots of stuff I like inexpensively, if I'm hungry on the spot or want to take it with me). Recently I found myself in Gatwick and saw lots of shops in the larger terminal, but when I hurried through to my terminal I found it was smaller, emptier and had bugger all of use to me. I was (unnecessarily) stressed about time and didn't want to backtrack, so powered on and hoped the duty free zone would have something like Wasabi - it did not, only Pret etc which I dislike. I resorted to a crap meal deal from Boots. Not the end of the world but totally ruined my travel zen. Don't do this, if you see a place that has food you want just buy it.
You can bring empty bottles through security and find somewhere to fill them. If you don't, Boots and WH Smiths have the cheapest bottled water.
Top tip if you can't find a seat in the busy shopping/food areas, or just don't like the crowds and noise and incessant announcements - walk in the direction of any random gate, find an empty one and sit down. Bliss!
Note that once you're out of the central pen the screens and announcements around you will not show all flights, only the ones from the nearby gates, so you'll need to wander back occasionally to check on yours. That's nice if there's two of you though, as one can stay with your bags while the other sticks their head round.