When the market is active (= more coin days destroyed) the value of a coin could deflate and vice versa. The value adjustability could be achieved by doing this calculation for a coin transfer:

for x amount of coins and y amount of coin days the receiver of the transaction receives clever_algorithm(x, y, market_activity_during_the_period_of_these_coindays) of coins.

This way the money supply can go up and down, depending on market activity, encouraging spending when the market is too inactive, and discouraging it when it is too active.

The variable that will become stable is the market activeness. Will this also lead to a stable monetary value of the coin?