Maybe for those that enjoy exercising, walking, hiking together but the Waterfall Gully to Mt Lofty walk is great. Its physically a challenge but you start at the cafe and waterfalls at the start of the walk and end up at the Mt Lofty look out with great views of the city and coastline of Adelaide and you can catch a nice snack and coffee there too, before descending with more time to look at the scenery.
I think that getting out into nature is always a winner! It's always nice (and can be very romantic) to take a drive around the Adelaide Hills or Barossa Valley, stopping at pretty villages on the way and taking walks around local; beauty spots.
The Adelaide Botanic Garden Mount Lofty Botanic Garden, and Mittunga Botanic Garden. You can enjoy with collection of plants from the island of Madagascar ,a beautiful selection of Amazon waterlilies, ornate Victorian detail, plants from the five Mediterranean climates around the world ,International Rose Garden and etc.