Victoria Park is the best for people watching! You could start walking from Bricklane (lots of vintage clothes shops - inc the massive Atika), or from Redchurch Street (little boutiques, plus Brat / Lyles, excellent restaurants), up Columbia Road, then along the canal to Victoria park.
Hampstead is a little more picturesque / impressive than Highgate, but both are great, and definitely visit Highgate Cemetary. Jin Kichi in Hampstead is one of my fave restaurants, great to sit at the yakitori grill - book as they get busy even during weekday lunch.
There's a v good walking tour of Hampstead - www.walks.com/our-walks/old-hampstead-village-walking-tour/ .
The southern bit of Waterlow park, right next to Highgate Cemetery is interesting on the weekends - it gets taken over by families doing BBQ's, which is a rare sight in London. Also worth popping into the charity shops in these wealthy areas!
If you're here in the summer or like cold swimming the Hampstead ladies pond is beautiful (need to pre-book from Spring). The mixed ponds are not quite as nice, but your partner could go to the men's ponds and you go to the ladies if you need a break from each other haha!
I'd say Clerkenwell is a fine base - Farringdon is on the speedy Elizabeth line. Lots of good food options - Bouchon Racine, Quality Chop House / Quality Wines, the original St Johns, Scotti's Snack Bar. Exmouth Market is a v cute street / good people watching. Also close to Sadlers Wells, and the Barbican (the architecture tour of the Barbican is interesting).
Primrose Hill has a cute high street, then you can walk down the hill and West along the canal to Little Venice. Along the way there’s Alfie’s Antique Market which is huge, full of character, and definitely off the tourist trail! Check opening times.
Portabello is v busy and touristy, however, the north bit around Golborne Road has less tourists and more character (and Trellick Tower of architectural interest). There are great antique arcades in basements on Portabello road north of Chepstow Villas, that the tourists don’t notice as they keep to the stalls outside. (arcades open on Saturdays)
The Museum Of Brands & Packaging is nearby, which I love for the nostalgia.
Use City Mapper for getting around, and don’t try to do too much in one day - it can get knackering!