For catching sand crabs:
as you look out at the beach/sand... you will see "holes" in it. These "holes" are the sand crab holes. That is where they live.
The holes come in all different sizes. Big = bigger crabs. Small holes= small crabs.
Then, once you pick a hole, just dig with your hands... or a toy shovel. Just keep digging... until, you feel or see the crab! Or the crab will scamper away. They are really FAST... and are hard to catch. So once you see them run away... take off running after them. AND have a net on hand... (the small hand held kind ), and then try and land the net on them to catch them in it. But if you lift up your hand/net too fast and not carefully, the crab will runaway. So, use your hand, to pinch t hat area of the net, to "grab" the crab.. .and then dump it in a bucket with a small bit of sand. If you put the crab in a bucket with just water... it will not have anything to dwell on... so put some smallish rocks in there too.
Next, another way to catch the crab is... as you are digging into the hole... keep digging following the trail of the hole... then, using your finger tips... you "feel" for the crab.... and examine each clump of sand that you dig out from the hole because sometime the crab will be in that pile of sand you just excavated. And, you can keep your net nearby... with the other hand... just over the opening... to prevent it from escaping etc.
Remember, the crabs are super quick. AND, they blend into the sand, because they are sand colored. So look carefully.
Don't just dig fast and furious... because the crab could be at any depth of the hole.. .and sometimes, they have another hole... the "escape" hole which is located elsewhere. So sometimes you will see the crab escaping out of another hole! Clever critters.
Anyway, its fun. I spent hours doing that as a kid with my Dad. And I taught my kids how to do it and they love doing it too.
They are harder to catch, in the water.
I am talking about the type of sand crabs I have in my State... one of many.
As for things to do with your 2.5 year old: well they get entertained with ANYTHING at the beach. Just play with the sand... have a floating thing/ring for her to be in ... if in the water, and safety things.
Sunscreen... and some use a little pup tent for shade at the beach too.
This is an example:
http://www.amazon.com/ABO-Gear-Portable-Beach-Shelter/dp/...
Don't worry about "entertaining" your 2.5 year old... because, they will naturally be entertained and just digging around and running around. And with that and being in the sun... they will be quite tired after, and probably nap well! Its just about being out in nature and finding things to do with what is there... picking up rocks, seaweed, catching things etc. No need for lots of beach "toys." That kind of defeats the purpose. When I was a kid.... we just "played" with whatever was there... natural stuff. Climbing rocks and running around.
good luck,
Susan