It didn't occur to me to pull the bass guitar
(a damn good one - I was so fortunate to stumble across it in 1979, and so fortunate to have never pushed very hard to sell it over the past 35 years - having not played it much at all for that long) out from under the bed, after several decades.
It was a guy who was hiring me to play a biweekly dixie job in a bar/restaurant who - hearing my bass lines - guessed that I had also been a bass player, and asked me if I would bring the bass to some subsequent gigs.
As far as "keeping the mind active" is concerned. Pushing my mind and body to move from C instruments to B-flat instruments (seriously, exploring what the "European orchestral tuba-playing model" is all about, listening to the top European players play the orchestral excerpts (linking arguably THE BEST one here in this thread), listening to them talk about playing on videos, etc., etc.) has kept me engaged...and I went back through ALL of the etude books (the difficult ones,
not the easy ones) over-and-over during ... lenged my mind (AND HANDS) far more than the C to B-flat thing has.
Those cats play/played a BUNCH of stuff, and CONSTANTLY. (They don't just come in during the loud parts - as do tuba players in symphonies - and play a few loud eighth notes, quarter notes, and donuts. Rather, those guys' playing is CONSTANT, BUSY, and FAST-MOVING. Even when repetitive, try playing the stuff they play over-and-over for three minutes at a time or longer (per their recordings). It's a lot of fast-moving, physically taxing, and mind-challenging playing (ie. "OK...how many times have I played this now, and is it time to move to that next thing?"...etc.)
"Chicago" transcribed bass parts...(Peter Cetera was/is a genius/wild man...and - curiously, I've since discovered - his equipment was very similar to my own)...those CERTAINLY push the brain and the fingers to WORK - particularly if those bass lines that he created are recreated WELL.

)
OK...If not pop/rock/funk/country/standards/etc. with electric bass, pick up a typical DOUBLE BASS ("string" bass) part to a typical symphonic work, transpose it down to the "sounding" octave, and play through it from start to finish (an overture, and entire symphony, whatever) with your tuba...busy, busy, busy...(more challenging that many tuba "solo" works).
...It's a bunch more fun to keep my mind engaged this way than to buy quack OTC "brain supplements".
...and for those of you who've not yet found this young gentleman:
https://www.facebook.com/profile.php?id=61577478992061