Just for posterity, here are the rest I came up with:

    'foobar' (e. ] A.~ +/@(!@i.@- *&|: >:@i.@- $&> i.)@#) 'barfoo'  NB. 5 - Musical Chars
    (/: #&>) 'books';'apple';'peanut';'aardvark';'melon';'pie'      NB. 6 - Size Matters
    (~. #~ [: (= >./) +/@|:@=) 'abdbbac'                            NB. 7 - Popularity Contest
    |.&.>&.;: 'a few words in a sentence'                           NB. 8 - esreveR A ecnetneS
    'foobar' #~ 1 0 0 1 0 1                                         NB. 9 - Compression Session
    'fbr' [`(I.@])`($&'_'@#@])} 1 0 0 1 0 1                         NB. 10 - Expansion Mansion
    '_'&(I.@e.&'AIUEOaiueo'@]}) 'FLAPJACKS'                         NB. 11 - C_ns_n_nts
    -.&'AIUEOaiueo' 'FLAPJACKS'                                     NB. 12 - Cnsnnts Rdx
    'one fish two fish' [`($&'X'@#&.>@[)@.e."0 _&.;: 'fish'         NB. 13 - TITLE REDACTED
    (A.~ i.@!@#) 'xyz'                                              NB. 14 - It's More Fun to Permute

Note that 14 is trivially non-recursive. I am fairly happy with these solutions, especially 5 and 13 which took the most thought.