Poon Hill is an awesome sidetrek option for a rest day with amazing views of Fishtail and Annapurnas and beautiful sunrises over the Himalayas.
If you stay in Ghorepani, you can wake up in the morning and do this side trek before you head out on the Circuit. It is approximately 45 minutes to get up to the top, so you need to wake up and leave by roughly 4:30AM depending on the time of year to make it for sunrise.
Plenty of other trekkers will be doing the same, so you will have no trouble finding it. When you get to the Poon Hill Trail, there is a 50 Rupee fee per person to enter, so make sure you have some money on you. They also sell hot tea and coco at the top for 100-150 Rupees. Make sure you bring a jacket, because it gets really cold up there before sunrise, also some water because its a pretty tough climb.