I just travel around at the beginning of may, nice area.
What kind of bike do you use in this trip ? It is a very nice road between Iacobeni (Bucovina) and Borsa (Maramures) but in a very poor condition. I was traveling with a naked bike (yamaha xjr 1300) and despite the nice view I was cursing non stop.
If you are interested in history (communist in this case) I also recommend Sighet Memorial in Sighetu Marmatiei.
Depending on when are you coming in Romania I suggest to check if Transfagarasan or Transalpina are open, normally both are closed until 1 july but if the wether is good they open them faster. Those to roads are the best way to cross the mountains and Transfagarasan is very close to fortified churches from Transilvania.
In Bucovina you can check also the painted monasteries.
I confirm the info about narrow gauge train from Viseu de Sus is quite an experience an you can sleep over night in the railway station in a train
All the best
ps. be aware that in Romania in some areas (most of them) there are street dogs. They are not dangerous (in general
) but they are barking at the bike. .. they are not biting, but can be annoying.