The key is to sustain a mentally challenging environment. Just riding your bike or watching TV all day won't do it.
Actually riding a bike (assuming one is outside) can be a very mentally challenging environment! Also, very recent fMRI studies have shown that exercise is very stimulating for the brain.
I'll definitely agree on the watching TV all day, though. Nothing makes me feel worse.
Meaningful "work" is important imho. MMM isn't sitting home doing nothing, but he is in a situation where he can decide at any moment to change activities. No one is forcing him to do dangerous or boring, repetitive labor or sit at a desk all day doing something he doesn't like (or, in the case of many 9-5er's, having nothing stimulating at all to do but sit there killing time.)
The most meaningful work is the work that you want to do, and many working people in paid employment don't have any flexibility about choosing their daily tasks. Self employed people generally have much more flexibility; in a way, being self employed is sort of like mustachian retirement, but without the stress of needing to keep going sometimes when you're ready to stop just bc you need the money. Reverting to doing nothing is surely a recipe for disaster for mental and physical health, but that would be a poor choice for how to retire.