Naviglio Grande is an historic canal, built to connect Milan to the Ticino river.
The area is one of the most popular among tourists. It has many bars and restaurants and the whole context with the canal makes it a very special place in Milan.
I like to come here in April/May and enjoy an Aperol Spritz in one of the bars along the canal and enjoy the sun. It's usually one of those things that makes me feel that summer is coming.
Navigli is certainly a must-visit spot, if you're looking for a peaceful walk, then a visit in the morning on a sunny day is the best (especially on a weekday). If you're looking to see the area buzzing then from 6 pm until late at night is what you're looking for.
Here there's also a vintage market on Saturdays, or in selected days. Check out "Navigli Vintage Market" to see if there's during your visit to Milan.
I used to come here a lot in the past, spent many nights out with my friends visiting some of the bars or just relax by the Darsena. I feel though that in recent years the area has become a little worse, especially late in the evening there are some sketchy people (even though it's generally safe to come here).