The Bugline is a crushed limestone trail (like most of the state trails.) While you could take a wheelchair on it, it would definitely be more work than a paved trail. The Glacial Drumlin Trail is paved from Waukesha to around Dousman, and lots of caches are on it. I don’t believe many of the caches are wheelchair accessible, however. This is, unfortunately, the norm for caches along bike trails. Because the traffic is so high, the cache need to be quite a ways off trail to avoid muggling.
Try the Hank Aaron State Trail — there are about 10 or so starting at GC1FH28 and ending at GC1JCWJ. GCQ7Z2 (west of the ball park) is also accessible, though GCA317 north of the I is NOT. Many of these are Earthcaches, so make sure your pencils and scorecards (and camera) are ready. GC1EBA is a short drive away and a good cache to finish up, especially M-Sa, 10-30 to 3:30 (factory is not wheelchair accessible but all else is). 😉 GC3798 is another classic apres cache geocache.
The other trails (Glacial Drumlin, Bugline, Milwaukee-Racine, Muskego) are old interurban lines. Very smooth and level, but caches are set back in the brush, often on cuts and fills, so not necessarily reachable.