Hi,

Weather in the main cities should be fine ( generally not too cold) and sunny. Autumn colours will have gone by mid December mostly ( start of December was excellent in Tokyo/osaka/nagoya)
I was in Osaka/Nagoya and Tokyo in December 2025 ( i posted daily photos in the trip report section)

Lots of christmas decorations/illuminations up in early December ( come down on 25th though some winter illuminations ( ie Tokyo dome city stay up till february)

Some ideas
Gardens and parks in Tokyo/Yokohama
Tokyo Disneyland ( they do christmas decorations very well)
Nagoya- lots of gardens and botanical gardens . The SCV Maglev/Railway park is very good ( also legoland Japan is close to the museum
Nabana no Sato ( beautiful winter illuminations) about 40mins by rail from Nagoya station. Also Inuyama ( just outside Nagoya ) Has a beautiful castle and garden
Lots to do in Kyoto
Lots of Gardens in Osaka ( very large Expo 1970 and 1990 Garden expo parks) and Minoh falls ( easy paved road up along a stream about 30mins by rail then 30mins walk from Osaka station). From Osaka it is easy to go to Nara ( lots of deer), Kobe and Himeji ( famous castle and garden)

Regards
TBS

PS- The Nozomi ( fastest shinkansen train) will be all reserved seats between christmas and new year. I would check opening times over the christmas period also