Tried and proven: paper (Score 1)

Use paper and encode your data using something like http://en.wikipedia.org/wiki/Data_Matrix or some other well documented data format. I think there will always be digital imaging devices like scanners and cameras around. Maybe even include decoder sourcecode in OCR friendly print. You're planning for 25 years, but who knows :-) Might result in a load of paper though... I wonder how much information you could squeeze on high resolution paper in multiple colors. As the capsule is bound to be perfectly dark the paper might stay in good shape.