Another "must do" is the boat trip on Maligne lake down to Spirit Island (Google Images - Spirit Island).
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Spirit Island is a must do. But not in May. The boat tours or rentals are not offered until mid June. If you have your own boat (not motor) you can go on your own. however, at the start of may, it is more tehen likely the lake is still iced. I did the drive to Banff last weekend and north of Lake Louise all the Lakes were frozen solid.
The Vermillion Lakes in Banff were the only ones that weren't frozen and are a good bet anytime of the year - great sunrise and sunset locations.
Best bet for early May in Jasper is probably shooting the Maligne River and the Road from Jasper to maligne lake. I've probably seen more diverse wildlife along this road then anywhere else (i.e bear, wolf, elk). Not sure about what you will see at this time of the year though (i think it is a bit early for bear sightings).
You can also check out Pyramid Lake and Mt. Edith Cavel. Again the Lake may still be iced.
But personally, I think if you can delay your trip by a month, you can have a much more productive time as many of the roads in banff are closed until after the May long weekend (i.e. Morraine Lake road), and most lakes are still iced until the at least mid-May.
At this time of the year, the Vermillion Lakes is probably your best bet as this is kind of the in-between times of the year, where there is not enough snow to get that wintery look but the lakes are still iced, so it also doesn't seem like spring. You will probably see quite a bit of snow along the highway from Jasper to Lake Louise as last week the highway from Lake Louise to Peyto Lake had as much as 5 feet of snow piled up at the sides of the highway from the snow removal efforts.