I'm looking for someone who is familiar with a great deal of classical
music to make some informed recommendations for further listening.

I'm citing Bach's Goldberg Variations as a piece that I like.  I hope
that someone can use that as a basis to make other recommendations.  I
enjoy the piece because it is simply orchestrated and because it has
repeated themes.

I would prefer recommendations to be works for a single instrument or
a group no bigger than a quintet.  (I've found I usually like these
better than full orchestras.)

An acceptable answer will contain at least 5 recommendations. 
Recordings of the recommended works should be readily available.

Let me know if any clarifications would be helpful.

Greetings, cope360-ga

The Pachabel Canon cited by Pinkfreud in the Comments section is a
good start for listening to a simple theme repeated.  But your delight
in the Goldberg Variations shows that you already have a sophisticated
ear and may wish to continue your musical exploratons with the
Masters.

Suites and variations have in common  the declaraton of a theme
followed by exposition of variations or embellishments on the theme. 
Usually, variations end with the statement of the theme as you know
from the Goldberg; suites do not.

While you will enjoy suites and variations of Bach and Mozart, I would
suggest you sample the sonatas of Scarlatti for harpsichord, if you
can get a copy.  Sonatas also build on a theme, but in a more
complicated way.  More complex, but still most enjoyable.

Jumping to the next period of musical development, the Classical, you
might enjoy Beethoven's string quartets.  The quartets of Opus 59 are
lush and dramatic.

Here are the recommendations of a music lover. I have noted a web
reference for each.

Bach Cello Suites 
http://www.amazon.com/exec/obidos/ASIN/B0000025QM/qid=1094090705/sr=2-2/ref=sr_2_2/102-7752968-9784948#product-details

Bach lute recordings 
 http://www.jsbach.net/recordings/index-solo.html

Bach English Suites  -piano
http://www.musicaloffering.com/Revu%20Goldbrg%209302.htm

Mozart variations -piano
 http://www.amazon.com/exec/obidos/tg/detail/-/B000007RXN/qid=1094090566/sr=1-17/ref=sr_1_17/102-7752968-9784948?v=glance&s=classical#product-details

Scarlatti Sonatas - harpsichord
http://www.amazon.com/exec/obidos/ASIN/B0000029SD/qid=1094091412/sr=2-3/ref=sr_2_3/102-7752968-9784948#product-details

Beethoven String Quartets (opus 59)
http://www.amazon.com/exec/obidos/tg/detail/-/630839288X/qid=1094092004/sr=8-9/ref=sr_8_xs_ap_i9_xgl14/102-7752968-9784948?v=glance&s=books&n=507846#product-details

Beethoven Diabelli Variations - piano
http://www.amazon.com/exec/obidos/ASIN/B00028HBPM/qid%3D1094092194/sr%3D11-1/ref%3Dsr%5F11%5F1/102-7752968-9784948#product-details

For a $25 annual fee, you can sample music from these and many other
composers through the Classical Music Archives website:
http://www.classicalarchives.com/main/a.html

Good listening.

Alanna-ga

Thanks for the great answer and the description of suites, variations, etc.

You might like some of the works of Johann Pachelbel:

http://www.kunstderfuge.com/pachelbel.htm

Reading your request, I'd recommend you continue to explore Bach.
Yo-Yo Ma's recordings of the solo cello suites are the gold standard.
Nathan Milstein recorded the sonatas and partitas for solo violin each
twice, I believe -- although personally, I'm partial to Hilary Hahn's
recording (including the partita in D minor, which is among Bach's
greatest works). Richard Goode recorded the solo piano partitas on two
discs for Nonesuch, and Eduardo Eguez released two discs of Bach's
lute music on MA Recordings (a label recognized for its impeccable
engineering). Of the many recordings of The Well-Tempered Clavier, I'd
recommend Angela Hewitt's (two 2-CD sets).

All of the above are solo music. I'll break from that theme for two
last recommendations: the Emerson String Quartet's recent recording of
The Art of Fugue, which absolutely meets your requirements; and
Karajan's recording of the Mass in B Minor, which does feature
expansive orchestration but which I think you'll enjoy nonetheless.

You may like Francois Couperin