If you want to spend part of your trip walking and observing, you can start with what the Jardim Botânico do Rio de Janeiro offers. The institution publishes themed routes and guided options. That information lets you choose a common thread for your visit, instead of trying to cover everything in a single outing.
Choose a theme that sparks your curiosity
The published self-guided routes include themes such as palm trees, giant trees, the Amazon, and the Atlantic Forest. Check the available materials and choose based on your interests. You don't need to know the names of the plants to enjoy an attentive visit.
One way to organize yourself is to choose the route first and then book the rest of the day. If you do it the other way around, you may end up rushing the visit to make it to another activity you had scheduled without factoring in travel time.
If you prefer a guided visit
Check which options are offered on your date, how to book them, and what each ticket includes. Don't assume the guided visit replaces admission to the garden: read the conditions of the option you choose. It's also worth asking about the language and about your group's accessibility needs.
Plan a comfortable walk
Check the forecast and wear shoes suitable for walking. If you're traveling with others, agree on a pace and on times to rest. Taking photos can be part of the outing, but also leave time to observe without worrying about completing a route.
What to combine it with afterward
Instead of adding another attraction right away, leave a break to eat or go back to your accommodation. Once you know how much time you want to spend at the garden, consider a second activity. That flexibility is especially useful if heat or rain changes your planned pace.
Source: Jardim Botânico guided visits and themed routes ↗. Confirm availability and conditions before booking.