Actually, coffee grounds are not all that acidic! The acidity is in the coffee that you drink, but used coffee grounds are only slightly acidic to nearly neutral. It's certainly not enough to change the acidity of the surrounding soil.
D, there are a couple of different methods of composting indoors or on your balcony. If you're not squeamish, you could start a worm bin. Another method is called bokashi, a Japanese anaerobic composting method that ferments the organic matter (without odor). But this requires you to bury the fermented product to finish the composting process, so you'd need access to a yard or a large container of soil on your balcony.
