So, no excuse, the weather's nice in Milan, and the civic museums are varied and sure to have something of interest for everyone. In Milan:
Museo Archeologico, a marvelous Goldilocks museum...not too big, not too small, it's juuuuussssst right...that gives you a good overview of Milan's ancient past and gives you a chance to see what remains of Milan's ancient Roman imperial city walls, this is always high on my "don't miss" list. See here for the my post written during a previous round of freebie entry. Go here for the museum's web site, alas, only in Italian.
Cenacolo Vinciano, aka da Vinci's "Last Supper"...one usually needs reservations for this...see my previous post.
Museo della Storia Naturale (minerals, dioramas, stuffed animals, informative didactics,...see my previous post; or the web site only in Italian)
Musei del Castello Sforzesco (the variegated museums in the city's lovely castle, including the collection of historial things Milanese and the ancient Celtic and Egyptian collections; web site in English)
Palazzo Morando (fascinating glimpses of the Milan of yesteryear seen in period paintings; the rooms are punctuated with displays of period dress; web site only in Italian)
Acquario civico (refurbished a few short years ago, the acquarium, similar to Goldilock's bowl of porridge, is "just right"...neither too big to bore non-enthusiasts, nor too small to bore fans...and there's a fun tub-tunnel...for a general look...scroll down...see this post)
GAM-Galleria d'arte moderna (go here for your 19th century art background before you head off to see the modern stuff...not so interested in 19th century art? GO ANYWAY...the early 19th century palazzo is gorgeous...stay out of the park in the back, though...it's reserved for families with young children; web site only in Italian)
Museo del Risorgimento (dedicated to the peninsula's finally successful attempt to become a single independent nation, it also has other delightful surprises: the coronation robe Napoleon wore in Milan's Duomo, the bell cracked by hammering during the 1848 insurrection, paintings whose content can surprise and move...; web site only in Italian)
A word to the wise: check out the individual museum's web site and/or call in advance to make sure that last minute problems or required advance reservations don't leave you disappointed.
Are you elsewhere in Italy? Then go here for the entire list...sigh, only in Italian...and be sure to note any caveats at the beginning of the page before you scroll down to find the region and city of interest.
Oh, before I close, do you come often to Milan, or maybe even live here? Don't forget that you can buy a museum card pretty cheaply that has a lot of benefits. See my previous post.