We arrive in August on Friday evening and leave Sunday morning. So far am planning on doing Tivoli Gardens, Rosenborg Castle and a canal cruise, along with possibly renting bikes. Any recommendations for additions (or subtractions)?
Any special interests?
Check events. Danes like outdoor events during summer:
https://www.visitcopenhagen.com/explore/events-cid58/events-cid59
My usual two cents:
Stay on the corner of Gothersgade and Rosenborggade at 11:27, when the guard leaves for Amalienborg. Follow them and watch the changing of the guard. Then go to Nyhavn for lunch.
Alternative canal cruise: https://goboat.dk/en
You seem to have enough options. Any more and you will be chock-a-block (jam-packed) up to your ears. Just sitting in a park for a while can be nice, to see what you can see, and hear what you can hear.
The canal cruise was great. Glad you are going on one.
If the weather permits, spend one evening at Tivoli (despite being an amusement park it has good food options) and one at Reffen street food market.