Being a huge fan of 60's pop and power pop, hi hat tambourine. I have the kind that mounts to the rod and the ching ring style that lays on the top hat, both Meinl, so they can cut through the rest of the band better. The only other percussion I have is a cowbell which I use for exactly two songs, Honky Tonk Women and Stuck in the Middle.
I'm thinking cowbell, though confess it's a new adventure for me. I had one for years that for the most part stayed in my case waiting for one of those few cowbell songs, which rarely came up. But when I hear some drummers work a cowbell into their semi-routine playing, I'm eager to experiment.
Second would be a wood bock. Those too can be worked in tastefully, and should be, but one thing at a time. Cowbell is my immediate adventure.