Oxford publishes a series of organ music for manuals only. Other publishers also do. Concordia used to have a very useful book called Ten Anonymous Eighteenth Century English Volutaries. If it is still in print, you might well find it very useful, not only for joyful Sundays, but for weddings and such. Concordia also has quite a long list of excellent not-difficult organ music.

Early English voluntaries by Gibbons, Tomkins, Byrd, et al., have the beauty of being quite admirable manuals-only music that isn't usually overly difficult. Later 'trumpet' and 'cornet' voluntaries by Stanley, Croft, Bull, Crotch, Alcock (mentioned above), Boyce, and others are quite nice for slightly festive occasions or on days when the lectionary has a really joyful theme.

Many of the continental partitas on hymn tunes, e.g., those of J.G. Walther and others are not too challenging and can serve to make one's voluntaries lectionary-specific.

The chorale preludes of Buxtehude make excellent voluntaries and are not difficult. Ditto some (but not all) of his free works, such as the canzonas.

Then, there are the famous eight little preludes and fugues by Bach (Krebbs).

Many of Frescobaldi's works are not overly challenging. Look into his toccatas, and the Fiori Musicali which has pieces which were intended to be played at mass, either as alternatim pieces or as fillers, for instance, following the epistle.

Pachelbel offers some very nice and impressive toccatas that aren't too long and that you could learn in one or two afternoons, as well as chorale-based pieces that are not at all difficult.

Too, you might find select versets from the French organ masses quite useful: Couperin, de Grigny, Boyvin, LeBegue, d'Aquin, Clerambault and numerous others. Many of these can be learnt in an afternoon or in a week or so. I often make a short three-'movement' eight-or-so minute prelude out of, say, a plein jeu, a recit, and a basse et dessus de trompette from a given set of mass pieces. You can choose such pieces that fit the mood of the day's lectionary and have a very fine musicalisches opfer!.

Then, for something truly modern but not daunting, don't be afraid of Daniel Pinkham's Five Voluntaries!

More later, perhaps. There really is no end of moderately challenging music suitable as ornaments of the mass for any occasion, mood, or season.

Wow some really great suggestions!
I should have been more specific and mentioned that I'm looking for piano or piano-friendly music, but a lot of this stuff will work. I've already done the Ciacona from Muffat's 'Apparatus' and it came off quite well with some creative pedaling, got many compliments on it which was surprising!

There are two books, 50 Baroque Fillers for Organ, and 50 More Baroque Fillers for Organ available from Kevin Mayhew. Both are mostly manuals only and are fairly easy to play. I use them often.

These are Couperin, Stanley, Frescobaldi, and similar filler. Good stuff! The longest works are a couple of pages, and judicious repeats can make them longer, if needed.

Ewww gross Adam LOL!!

I am unemployed. You can only look for jobs so many hours a day.

Don't overlook Sir Charles Villiers Stanford's Preludes and Postludes, the first set of which is available online here:

http://javanese.imslp.info/files/imglnks/usimg/4/4a/IMSLP15512-Six_Short_Preludes_and_Postludes_Op101.pdf

I quite often use Number Six, in particular, which is very obviously based on the hymn tune that most of us associate with the words "Oh breathe on me, oh breath of God."

The best short preludes OF and FOR all time:

http://imslp.org/wiki/Liber_organi_(Dalla_Libera,_Sandro)

Many if not most are sight readable without practice.

More ideas on these threads:

Beginner Organ Music

Postludes

Organ Preludes and Postludes