Which side of the park will you be staying on?
Some short hikes with small elevation gains:
Logan Pass (if open)
Hidden Lake - morning is better, if only for parking, usually plenty of mountain goats and often has bighorn, too.
Highline Trail - not for anyone who has a fear of heights
West (from Lake McDonald)
Avalanche Creek and Avalance Lake - The lake is about 2 miles from the road on a relatively flat hike. If you can't make it to the lake, the creek has some great photo ops.
Haystack Falls - an unmarked pulloff on GTSun road, near Triple Arches, below Logan Pass, may still be buried in snow. This waterfall looks like a series of hay bales stacked one on top of each other.
North East (from St. Marys)
Sunrift Gorge and Baring Falls - both are in shade most of the time. The gorge is a few hundred feet to the North of the road; the falls are .3 miles to the South of the road. Don't forget to check out the creek as it passes under the road.
Sun Point - great for sunsets
South East (Two Medicine)
Running Eagle Falls - .3 mile, morning until early afternoon, also called "Magic Falls" since the water can appear to be coming out of the cliff face.
My wife and I went last year and attended an
Exposure36 workshop and intend to return in the not too distant future. Let us know how the ballon ride turns out - sounds like fun!
It looks like you'll have a good selection of focal lengths to play with. Even though the wildlife is very used to humans at Logan Pass, the long lens will come in handy. Oh, and don't forget your tripod! The waterfalls beg for long exposures.
Finally, make sure that you pack or purchase bug repellent! Texas' mosquitos and horse flies are wimps compared to those up north.
Best wishes,
Steve