We just went to Ardastra Gardens ourselves since my kids 3 and 7 at the time would not have been interested in the city tour. It was really great!
The gardens were simply beautiful!! We spent about 4 hours there just wandering around, feeding the lorikeets (a highlight!!), finding hidden trails to see flowers and animals, walking with the free range peacocks and flamingoes, holding the parrots and watching the March of the Flamingoes show (my girls were chosen and it was so cool!)
The taxi was $20 with tip, each way (we haggled a bit). The zoo/gardens accepted a discount (I think it was Discover, but could have been AAA) so it was all extremely reasonable.
We had gone years ago and they charged extra to feed the lorikeets and hold/photograph the parrot. This year it was all included in the price. We must have hung out with lorikeets all over our arms, head, etc for 30 minutes. It was amazing!
I do want to try BL next time since you all are giving it such wonderful reviews. We were also there about 18 years ago, so looking forward to seeing it now.