If it wasn't for the forest, Rotorua could well be a mecca for roadies.
The loop around the lake, done in a counterclockwise direction, is a great 42km journey. It has a wide enough shoulder to ride on the left side of the white line almost all the way around. The only part that requires riding on the right of that virtual boundary is the section on the water after you turn off the main highway and take the road to Hamurana. It is only a kilometre or so, pay attention to traffic behind you and navigate that part with your full concentration. No earbuds!
You can extend the lake loop by climbing Te Waerenga Road, and heading out into Kaharoa, or climbing Oturoa Road and cutting across to Mamaku and back down Dansey Road, or you can make a 65km ride out of the lake loop by adding Paradise Valley, which is also a really nice shorter ride from town in either direction.
Heading south out of town and taking a right on to SH30 opens up Waikite Valley. You have a fairly dire section of SH30 to negotiate first, with a wide shoulder but fast moving traffic. There is a short section with no shoulder, please pay attention!
Not far up SH30 you will come to Bryce Road, which is a much quieter road leading to a wide variety of possible loops - it really is a follow-your-nose, you-can't-go-wrong sort of place. It's easy to rack up over 100kms out that way.
For people who prefer an off-road experience (we're talking 'gravel'), there are also many options.
Peeling out of the mountain biking car park on a gravel bike is not a bad idea for a start. There are many kilometres of forestry roads within the forest, and some of the trails can handle a gravel bike too.
Further afield, the roads to the north of lakes Rotoiti, Rotoehu and Rotoma make a great place to explore. The extremities of Waikite also feature unsealed roads, take a map and get lost!