overnight bus/train in East Java and Gunung Bromo

Bromo is best visited via Probolinggo, not Malang.
And Probolinggo is much closer to Bali, too.
No need for a tour - just go by public transport from Probolinggo to Cemero Lawang, which is right on the rim of the Tengger Crater.
One night there is enough for most people, but two would allow exploring the area beyond the standard viewpoint and Bromo climb.

Dieng is definitely worth visiting, and definitely with an overnight stay.
There is too much to see there for a day-trip.

Hi Alex10,

I have visited Bromo from Malang. It takes about two/three hours by bus to get to Probolingo. From there you can take a minibus up to Cemorro Lawang. I arrived there at 22.00 and left next day at 15.00. I hiked up a mountain in the morning and took a horse-ride during the day and that is about it. It is spectacular but there is no need to stay longer than 1 day.

PM me if you need more info.

and by the way...

make sure you get to Probolingo on time to get your minibus to Cemore Lawang. I got there at 16.00 and ended up forking out 70.000 rupiah because I was the only one going up the hill...