If you are in Summerside, I would start with the waterfront for sunset. The Confederation Trail where Heather Moyse Dr. meets Queen St. offers a nice view of the water and the Indian Head Lighthouse in the distance. On some evenings, you’ll also be able to watch some wind surfing.
This time of year the sun is setting more to the north and that will make it a bit tricky from Summerside on the south side. Look for an area that doesn’t have many trees to the west. By September and October, the sun begins to set more towards the south making anywhere along the Summerside boardwalk and MacKenzie Dr. are perfect options.
If you would like to see sunset over the ocean, perhaps Cabot Park in Malpeque (or any beach along route 20). It would be about a 30 minute drive to the north shore.
The Cape Egmont Lighthouse is also a 30 minute drive and would be a great place to watch the sun set over the Northumberland Strait.
And I would consider Chelton Beach Provincial Park or the Seacow Head lighthouse in Fernwood. Both are approximately 20 minutes from Summerside and offer unobstructed views of the sun setting over the water.
All the best. And enjoy each and every sunset.