Fruits and veggies are awesome additions, but a few others that I use are:
2 tablespoons of oat bran (59 calories, 3g fibre) that can be added to cereal, yogurt, peanut butter, mac and cheese - anything, really!
2 tablespoons of wheat germ (45 calories, 2g fibre) - same as above.
Popcorn! (I use the Orville Redenbacher's Smart Pop which is 120 calories and 7g fibre per serving)
I also make and freeze my own high fibre muffins, instead of using store-bought cereal bars. Although they take a bit of time to make, I do big batches once every month or so, so they're always available. I find that they taste better, have more fibre, have more protein, have more iron, and have less fat and sugar than the bought stuff. There are some great recipes on the sparkrecipes site for both muffins and protein bars that you might really enjoy.
Just remember to increase your fibre intake gradually with lots of water --- your body might take a bit of time to adapt to the increase.
Good luck finding something that you'll enjoy!