One of the best places you can visit in Varenna is Hotel Villa Cipressi. This hotel is a great place to stay if it's something you can afford, but if not, you can still visit it's beautiful botanical garden.
I'd recommend purchasing the ticket that includes both Villa Cipressi and Villa Monastero botanical gardens, they are both stunning and next to each other. If you plan to visit Villa Monastero as well, make sure to purchase the tickets in Villa Cipressi, as it's usually less crowded and it will take less time to get a ticket.
This botanical garden is very scenic and it offers some of the most incredible views you can see on Lake Como. I highly recommend it as a top thing to visit in Lake Como. Even if you have only a day this should definitely be on your bucket list.
Best time to visit: my absolute favourite time to visit Villa Cipressi is mid-April, since the wisteria will be in bloom and there's so much wisteria here! A whole wall covered in wisteria, which makes it so pretty to see around this time of the year.