For a gift card, you likely can't beat Harbor Freight for value. There's always something useful there, and things go on sale etc. so there's a good time and a not so good time to buy the particular thing he might need.
Another nice, inexpensive thing to do is paint on the walls. Not sure what your situation is, but I know in my case I could really use a couple days in the garage to take EVERYTHING out, paint the walls, and put everything back in. Would really brighten up the space. Sadly I don't have a 100 year old barn with awesome old beams and planks to look at. Just plywood on the insulated walls, so a coat of white paint would be better than what's there now.