As krp mentioned the May long weekend is still still early spring like weather for this area. I hiked the Lake Agnes hike in late May - stepped 2 inches off the snow packed trail and ended up hip deep in snow. It took two friends to pull me out. So if you do want to "hike" this area I would suggest bringing Snow Shoes :).
Not to diswade you from hiking...there are some nice trails - just be prepared. Johnston's Canyon is good anytime of year...weather dependent...if it has been a cooler spring take Crampon's with you. Or something you can put over your shoes to keep you from slipping in the icier areas of the hike.
The hard thing about travelling this time of year is the weather conditions - a late snow storm can shut down the Icefields parkway ... usually only for a day - but still can. So be prepared to alter plans. It is not usually busy - so not making you accommodation plans until a couple of days before would not be a bad thing. Last minute hotel deals are usually easy to get - and sometimes could allow you to get some really amazing prices at some of the more expensive hotels.
If you want a "cool" experience and you are going to be in the area for the Long Weekend - the Slush Cup weekend at Sunshine is actually pretty fun. There is a band playing on the hill, people dressed in all kinds of costumes - or in some cases - undressed :) skiing down the hill to jump a "pool" of water at the bottom of the hill. They have BBQ's out around the hotel for hot dogs and burgers. And you can take the Angel Chair up for sightseeing - and if you really want to ... you can ski :)
For Accommodation - depends on what type of experience you want for your stay. I think B&Bs are nice and cheaper alternative to the larger hotels - UNLESS you want the hotel pool, room service experience. Especially on weekends I have found the B&Bs are just a little more quiet than the Hotels. The B&B I stay at in Banff is Mountain Home B&B - the hosts are amazing and the locations is perfect. Just a couple of blocks from downtown Banff - all the restaurants and shopping...but still on a quiet street. Hotels - Banff Park Lodge is a nice option for location close to downtown - The Rimrock Resort - right next to the Gondola and Hot Springs is convenient as well...just a few minutes drive to the downtown area....or if you are able to get a last minute deal staying at the Banff Springs would be a real experience.
Canmore Accommodation - You can get a bit more bang for your buck. I have not stayed in the B&Bs there - it is outside the park - so a further drive to Lake Louise etc. But still doable. They have some hotels - though most of them are older...but nice option is some of the Condos in the area - you can get a studio room or 1 bedroom condo with kitchen - in case you don't want to eat out for all of your meals... for about the same price as a Hotel room in Banff.
The Trip up to Jasper along the Columbia Icefields Parkway is STUNNING. Definitely have your camera ready. Problem is - most of the Lakes - Lake Louise, Bow Lake etc....all the beautiful "BLUE" lakes will probably still be frozen, or just starting to shed their ice. Again it depends of course on the type of spring the area has had. It isn't unheard of for the Ice to shed in Mid May - if they have had a warm February March and April...without a lot of snow...but if they have received a lot of snow in the spring - then all that has to melt before the ice starts to....SOOOO... your pictures might not be what you were expecting. But still quite beautiful.
Jasper itself is at a lower elevation than Banff and Lake Louise - so that area may have less snow - so a bit easier for hiking. There is Maligne Canyon which I think is one of the Must Dos.
And the great thing about Late May - a lot less tourists to get in you pictures :)
Good Luck and hope this is of some help.