Already mentioned, but only in passing; the Griffes Poem is a gorgeous piece, and though it isn't cutting edge it's still got a very modern sound, and should be looked at if only for the opening melodic line (which is amazing - the phrase never actually ends until the end of the piece). It's got a lot of contrasting sections, so definitely something to look into if you want to show off - it has slow, moving sections, quick, furious sections, a jig in the middle, and a cadenza that has possibly one of the most irritating collection of notes I've ever come across, a real beast to play quickly.
Kennan's Night Soliloquy does have an arrangement for flute, orchestra, and piano, and while it's not precisely tonal (the entire piece is extremely unpredictable), it's still got an amazing cohesiveness, and it's tonal enough to sound good to someone who doesn't like atonal music. If you can play really delicately, this piece would be great.
Hanson's Serenade for Flute, Harp, and Orchestra, while what, 60 years old, is also one of those pieces that's definitely not classical, but also isn't scarily modern. It's very tonal, almost too tonal, but it's also very free. I wouldn't consider this extraordinarily difficult, but to sound good on it you're going to have to have a firm grasp of musicality or sound like mush. Worth looking into because it's so beautiful, even if you're not going to play it.
One more - Concerto by Otar Gordeli is probably more what I would consider 'modern classical', if that even makes sense. It's definitely not Mozart, but at the same time I think it sounds like something Mozart might write if he was alive today. Amazing cadenza, nice contrast between slow and fast sections, and tonal.