An example of developing a short simple melodic idea - the first of Rachmaninov's Symphonic Dances:
The score (full orchestra as well as arrangement for 2 pianos by Rachmaninov himself) is available on imslp.
Another fairly simple example - one of Dvorak;s Slavonic Dances
(the orchestral version was used in the animated film Allegro Non Troppo )
More interesting examples.