Explore your neighborhood. Before it got scorching here in Houston we were spending hours every morning exploring, and sometimes getting lost, around town. Birds, trees, insects can change from one street to the next - you will be surprised at the diversity you can find just by leaving your street.
Find nature sanctuaries - most big cities have these "green spaces" - ask locals, find birding groups, homeschool groups - all good sources for finding these spaces.
Study the weather!! Check and chart the temps and barometric pressure. Watch the clouds. Sketch the cloud formations along with the temp and barometric reading - study how they are related. Take the kids outside when a front is living in and let them feel the temperature drop, the electricity in the air.