Gioachino Rossini (1792-1868) was a very prominent Italian composer in the ninteenth century. Although some of his most popular works were writtin in the Romantic period, Rossini also wrote works in the late classical style. His Sei Sonate a Quattro (1804) demonstrated Rossini's early compositional style in the Classical era.
A link to the full score can be found at Rossini: String Sonata No. 1.
What placed this composition in the Classical era and not the Romantic was the overall structures of this piece. The form and phrases abided by the rules set forth in the Classical period. The phrases generally stick to four bars, and the harmonies and cadential figures were very predictable. The overall texture of this piece was relatively thinner compared to other later pieces by Rossini and other Romantic composers.
Written in 1845, Rossini's Bassoon Concerto provided an excellent contrast to his String Sonatas. While this piece still retained a certain level of predictability, the chromatic harmony shifts were a dead give away that this piece was no longer in the Classical period. The overall texture of this work (orchestral and not soloist sections) were thicker than that of the Sonata. The cadential figures were still treated in a "classical style" in the concerto, but they were sometimes altered with an extension of harmony or even chromatic shifts.
I chose to do Rossini because his earlier works are not as memorable or played as often (it certainly is not the Barber of Seville). Rossini did begin his compositions in the late classical and there is an audible difference between his early and late styles. However, I am not sure I can forgive Rossini for leaving out the violas in his String Sonatas.
Gossett, Philip. "Rossini, Gioachino." Grove Music Online. Oxford Music Online. Oxford University Press, accessed April 5, 2014. http://www.oxfordmusiconline.com.proxy.lib.utk.edu:90/subscriber/article_citations/grove/music/23901pg1.
Rossini, Gioachino. "Gioachino Rossini Concerto in B Flat Major for Bassoon and Orchestra" (video). Accessed April 5, 2014. https://www.youtube.com/watch?v=x1tkli_TLNs.
Rossini, Gioachino. "Gioachino Rossini - Sonata No. 1 for Strings in G Major" (videeo). Accessed April 5, 2014. https://www.youtube.com/watch?v=1Jejwa7l9v4.
Rossini, Gioachino. "Sei Sonate a Quattro." http://conquest.imslp.info/files/imglnks/usimg/b/b5/IMSLP105550-PMLP176737-rossini_string_sonata_1_score.pdf.