Some of it has really neat red streaks inside and those are used to make ornamental boxes. You'd want to seal it because it can have an unpleasant odor when wet. It is also fairly allergenic so if you get congested or a rash when working it then maybe it's best not used for woodworking. It's very porous and breaks down quickly so I think it may be pretty good for hugelkulture beds, but maybe not if it would draw too much nitrogen from the soil.
Possibly the best use, aside from making maple syrup, would be to chip it and use it to kill weeds somewhere - if it quickly broke down then you could get rich soil in no time, especially with the addition of manure or lots of chop & drop vegetation. It may also make good litter under rabbit cages, somewhere the animals aren't in contact with it (because it's allergenic), because it would use the extra nitrogen to break down really fast, making a wonderful garden topper.
Goats and other browsing livestock really like to nibble the smaller branches and I think would like to forage coppiced elders.
Some of this is based on info. from the
wood database:
http://www.wood-database.com/lumber-identification/hardwoods/box-elder/
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