Nandi Hills is best at time of Sunrise, therefore you should leave Bengaluru early morning so that you are at foot of hills by 5:15 (before the entrance is opened by cops). This is too early but considering the rush during weekends if you delay you might be stuck in traffic firstly at foot hills, then in queue for tickets, by then you will miss the beautiful view that can make your day. From dawn till sunrise and few more minutes enjoy the nature, take photographs followed by breakfast at Shops on Hill Top. By then It will be 8am approximately and you can head back to Bengaluru or move ahead to explore nearby areas (historic temple, flower gardens, Avalabetta). For Bike rental, there are plenty of options in Bengaluru. I will suggest you to pic from hub near to your home. Check out Drivezy, Onnbikes, rentnzip, Wheelstreet, Bounce, Vogo, Metrobikes, Royalbrother for bike rental. For any query or more details you can connect me.