Even for those who don't sing lyrics, singing solfeg is a great mental exercise, especially on Bach. Bach Fugues even more so. For an afternoon of fun, get together with three of your friends, two pianos, and play and sing each part. Assign the soprano and tenor to one piano, and the alto and bass to the other. It's a really cool experience, one that whoever hasn't done it should try.
You really get a sense for when each subject comes in, and plus it makes your mind more agile at the same time!