Hi Trinity,
Without knowing the ages of the children, it's a bit hard to make suggestions. That said, I suggest activities that require a lot of time and effort, regardless of mental ability, such as:
*Digging for fossils.
There are fossil sites all over the US and you can also buy "fossils" embedded in plaster from a company called GeoSafari. Digging them out takes a long time, and my sons have blast. BUT kids need to be at least 5, maybe older, and supervised depending on their ages. **The kits have sharp tools.**
See:
http://www.discoverthis.com/trex.html *Beachcombing.
This is fun if you live near an ocean. Kids (and adults) can learn how to identify the various shells and bones that wash up on the beach and how to preserve them. We've started a fossil-and-beach museum with my 2nd son who's a mad zoologist/paleontologist.
*Home astronomy.
You can begin to explore the universe with a pair of binoculars. A small- or medium-sized telescope is not too expensive and will provide stunning views of the moon and the nearer planets. Look at the moon especially when it isn't full. The areas near the terminator appear in fantastic 3D relief.
*Stamp and coin collecting.
Hope I've helped.
Val