Here are some recent  Word-of-the-Day entries that I think would be good for (upper levels) of Lexercise: 

1)   matriculate \muh-TRIK-yuh-leyt\, verb:  To enroll in a college or university as a candidate for a degree.

2)  eidetic   \ahy-DET-ik\, adjective:  having a photographic memory for  visual images

4)  utopia   /yoo-TOH-pee-uh/ . noun:  An ideal place or state, often unrealistic

 The r-controlled vowel sounds are easily confused by clients so we could use some more words contrasting them.  Here are some minimal pair words contrasting  
      <ar> versus... /ɝ/  spelled [<ur>, <ir> or <er>]  

at Level 15
arc irk
barley burly 
barn burn
Bart Bert 
barn burn

card curd
Carl curl
cart curt
dart dirt
far fir  fur
farm firm
farmer firmer
farthest furthest
fast first
lark lurk
mark murk
par per purr
parch perch
park perk
part pert  

quark quirk
scarf scurf
shark shirk
spar spur
star stir  

At Level 20: person, parson

At Level 24:purse, parse 

Searching for "squa " , "sque" and "squi"  in the data base (using [Ctrl + f]...... I find these words:

At Level 25 (because that's when this is addressed:  The sound of <a> shifts to /ɑ/  after the phoneme /w/)

squawk 

square

squelch

squall

squash

squat

At Level 11 :

squish

squid

At Level 13 (initial & final consonant blends)  there is:

squelch

squinch

squint

At Level 15 :

squirm

squirt