If you walk through the village then you will automaticly hit a walking trail that goes fairly steep uphill.
That trail leads you in to the Helambu valley where you have excellent trekking trails.
When leaving Sundarijal, most people walk to Chisopani the first day, but you can continue further if you are fit.
Just be ware that you are entering the Himalaya range and you should be resonably fit and well prepared before undertaking a trek there.
Sundarijal is connected to the outside world with bus and you can easily travel to and from Sundarijal from Kathmandu.
Not all busses from Sundarijal go all the way to Kathmandu though and you might need to change bus in Jorpati, but this is a simple procedure.
The busses stop in the main street in Sundarijal so you can not miss them if you need to take a bus from there to Kathmandu.