Hi !!
The full day bike rental is a great option. One can hire a bus, metro or train but that is good for a final destination. It's always better to hire your own vehicle to explore the city. Bike is the best option as it is light, convenient, pollution free and very friendly experience to ride and stop as per your wish.
We collected our bike near Colosseum and drove it till Vatican and back. Nice roads, pleasant weather and helpful people made the bike road trip more adventures. I would recommend this to everyone.