WomAdelaide I think falls in that time frame, or the Fringe, so maybe walk Rundle Mall, there will be performers doing their acts.
If you don't hang around too long each act, you don't have to pay.
Yes, the Botanical Gardens is good, walkable, as Adelaide CBD is flat(tish) only with a slight hill going towards St Peters Cathedral North Adelaide.
Maybe the Adelaide Zoo?
Or take a bus or train up to Belair Conservation Park in the Hills?
Trip on the tram to Glenelg.
$30 of the Adelaide Metrocard Adult non concession can last maybe 7 days, there is not a weekly pass, but that $30 will reduce slowly per use.
Get one from the Adelaide Metrocard office on the corner of King William and Currie St. You do pay an amount to buy the ticket, there is the option of paying cash per trip on the buses, but you only get a papery ticket for this, and this paper ticket last more than 2 hours, daytrip last all day, but with each insert into reader this can bend and make the magnetic strip unreadable.
Adelaide Metrocard, you just tap, it warns you when your "funds" are low.
Aud$3.48 for times before 9am, and after 3pm, or Aud$1.91 off peak, 9am to 3pm, each ride last 2 hours, Sat all priced $3.48, Sun all priced $1.91. Reach from Gawler in the north to Seaford in the South, from the sea to Mt Barker in the Hills.
Good for Mon to Sun trips.
There is a monthly 28 day pass, but as you are only here for 21 days, up to you to get the monthly pass, or per use ticket.
Private bus wine tasting further out, either up Gawler or down Sellicks way.
Beerenberg?
Kangaroo Island?
Fly to Port Lincoln for shark cage diving? $$$.