Was there for a couple of day trips recently whilst staying in Como.
Como itself is worth a trip – about 40mins on the train.
If Spas are of interest then this is rather nice: https://www.qcterme.com/en/milano/qc-termemilano
As has been said, even if you’re not a car fan, the revamped Alfa Romeo museum is gear – even Mrs Nobby liked it & she’s about as far from a car person as it gets.
The San Siro might be worth a visit as they’re retiring the old girl – it’s iconic but only really for what’s gone on in it over the years.
Even if you’re not going to use the trains, Milan Central is a fascinating station too.