Whole grains, fruits (I shouldn't have to say it, but not the juice, and if you do drink juice have pulp), beans and lentils, and leafy greens. These are all things Americans don't eat enough of for some reason.

Bean tacos are amazing. Black bean burgers are one of the few vegetarian alternatives that havr gained popularity among meat eaters, and for good reason. Have a spinach salad or snack on kale chips. It's hard to find something more nutritious than lentils though they're hard to cook in a particularly appetizing way. Cabbage and other brassicae can be a challenge but when cooked well is amazing. Whole grain bread takes some getting used to but is excellent once you do.

And if all else fails, pscilium husk fiber supplements.

The problem with fiber in the American diet is that it's satiating and noticeable in ultra processed foods. It's most appetizing in it's natural sources, and the American diet really isn't conducive to cooking with whole plant products.