Does anyone have a suggestion for a (paid or free) sans-serif family with small caps (and ideally old-style figures) that would pair well with Garamond Premier Pro? (It has caption and display sizes, so I'd prefer not to switch.)

Sans-serifs with those attributes are not typical. Still, I've found Futura Next (in the top spot so far), Dexa Pro (a close second), Alegreya Sans (probably better paired with Alegreya), Artegra Sans (no OSF), Source Sans (hmm), and Fontin Sans (not an ideal pairing).