These are some of the most popular classical pieces of all time!  I have selected them specifically with children in mind, so all should be enjoyable to adults as well. 

Owning music like this is always a good thing, but don't forget that your local library is also a fantastic resource to find these popular pieces.  Try to actively listen to 1 of these pieces a week.  Ask your children, "What do you think of when you hear this?"  This will help them listen more attentively, and imagine more vividly.  Happy Listening! 

Relaxing...
Air on the G String--J. S. Bach
Polovtsian Dances, Prince Igor--Borodin
The Four Seasons, spring, summer, winter--Antonio Vivaldi
Adagio for Strings--Samuel Barber
Symphony No. 9, 1st movement--Dvorak
Romeo and Juliet Fanatasy Overture--Tchaikovsky
Clair de lune--Debussy
Nocturnes or Preludes--Chopin
Rhapsody On A Theme Of Paganini, Op. 43 (the 18th Variation is everyone's favorite)--Rachmaninoff
Gymnopedies (3) for Piano--Eric Satie
Canon in D--Pachabel
Peer Gynt Suite: Dawn--Grieg

Exciting...
Flight of the Bumblebee  Rimsky-Korsakov
Bolero--Ravel
The Barber of Seville, overture--Rossini
Symphony No. 5 in c minor--Beethoven
Peer Gynt Suite, No. 1, In the Hall of the Mountain King--Edward Grieg
Etudes--Chopin
Piano Concerto in F--Gershwin
Rhapsody in Blue--Gershwin
Entry of the Gladiators (Einzug der Gladiatoren), march for orchestra, Op. 68--Fucik
William Tell Overture: Finale - Rossini
Gayaneh (Sabre Dance) - Aram Khachaturian

Majestic...
The Planets, Mars--Gustav Holst
Also Sprach Zarathustra--Richard Strauss
Mars (The Planets) - Holst
1812 Overture (Festival Overture for Orchestra in Eb Major, Op. 49, Finale) Tchaikovsky

Entertaining...
Carnival of the Animals--Saint-Saens
Peter and the Wolf--Prokofiev Peter and the Wolf includes a tutorial on what instruments represent what characters.  Very good to listen to even online.
The Cat and the Mouse--Bernstein
Caucasian Sketches (Procession Of The Sardar) - Mikhail Mikhaylovich Ippolitov-Ivanov

Good for the Brain...and Fun to listen to!
Water Music (Music for the Royal Fireworks) --G. F. Handel
Eine kleine Nachtmusik--Mozart
Symphony No. 40 in G minor: l. Molto Allegro--Mozart
Concerto for Piano and Orchestra In A Minor Op. 16: Allegro molto moderato --Grieg
Variations on "Ah, vous dirai-je maman" (Twinkle Twinkle Little Star) K. 265--Mozart