I could not speak to value for the money, as I sense everyone’s budgets vary, but you might consider the Yucatán. Although technically in North America which you did not specify, it is east of the Isthmus of Tehuantepec and thus unique from the remainder of the country. Depending what you wish to see (birds, insects, reptiles, flora, all of it), any number of excursions would work but honestly the nature is everywhere here. edit to add: Pronatura Península de Yucatán, which is a local conservation organization, has an abundance of images on their website showing natural areas: https://www.pronatura-ppy.org.mx/
For example, CICY (here is an English link) has a botanical garden where you can see numerous insect species in an extremely accessible setting, as well as learn more about the plants before heading out.
The abundant ruins are wonderful for seeing reptiles, large and small, who seem to bask on the stones. Some of the more accessible ones (IMO) are Uxmal, Dzibilchaltún (temperature dependent), Mayapán, and Xcambó, which is on the coast.
The coast also provides access to coastal species, including flamingos and other coastal birds. There are numerous guides available but one with an excellent reputation is Luis Trinchan, who is also here.
The state also boasts mangroves (spelling?) in Celestún and you can travel through them by boat with a guide. I have not done this yet but have friends who have and very much enjoyed this.
There are also cenotes here which have strange pale fish which species I cannot identify but those you will only find here, so they are a nice thing to see. Accessibility for many can be problematic however they are extremely well documented so research on the front end would be the solution.
As far as protected areas of nature, here is a map showing the current ones.
I do want to mention that mobility is an issue here for our residents who utilize wheelchairs largely because sidewalks are the responsibility of property owners rather than maintained municipally, so they vary in height and surface composition from house to house. Additionally historic buildings are attempting to become inclusive but ramps are often too high or doorway width is not expanded. That said, people here have extremely good hearts and will always pitch in to assist (solution finding is very much a thing here) but I do want to mention that.
As far as observations go, you are welcome to look at mine but they are all made in my own garden. (Did I mention nature is everywhere here?) I might look up some places you could go here (or anywhere!) and look through observations there.
Best wishes as you plan your exciting trip!