Is there an area you can get outside with the kids? Will you have another adult helper? Then plan to go outside to different areas nearby (not a playground) and explore, with paper and clipboards in hand to draw or jot notes about what they find.
Also take time to gather some nature materials you can use on a day when the weather won’t allow you all to go outside – such a rocks and minerals, pinecones, different leaves that you laminate/put in wax paper/preserve somehow, owl pellets, etc.
Maybe have nature walk inspiration sheets that list things to look for in very general terms – find something interesting that is green. Do you see any flowers? These prompts can give them a place to begin a notebook page on days when nothing is really standing out to them as interesting nature.