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There are half a dozen Trio sets that purposely don't have any theme at all. The instruction manuals you get with most of them do make some suggestions as to what you can do with the pieces, but you might want to just set them aside at first and let your (child's) imagination take over.
If you want to quickly check the pricing and availability of these Trio sets at Amazon, click (tap) the links below.
Getting Back to Basics or Starting at the Beginning
The first general set that Fisher-Price created for Trio was simply called the Basic Building Set (P6837).
You get 86 building pieces (counting the 2 "bonus" pieces) in a complete set. As you can see from the picture above, you can do a lot with that many pieces.
A complete list of those pieces follows.
A Storage Box: An Awesome Idea
Look at all you get with this basic Trio building set! And you get a storage bin that doubles as a base plate to boot!
Note how the lid of the container has ridges and holes just like Trio bricks. That's what I mean when I say you can use it as a base plate on which you can start your creations. Leave it on the bin or set it on the floor, whatever make the most sense to you.
You get 99 building pieces (including the bin and lid) in this set. You might see some people refer to this as a 100-piece set (a nice round number), but that includes the instructions in the count.
Note too that, even if you put everything into the bin, there will be lots of empty space inside. It might look like you didn't get a complete set, even though you really did.
Here are those 100 items you should expect in a complete set.
Pastels for Everyone and More Storage Too
Fisher-Price must have thought that their Trio sets were appealing mostly to boys, so they came up with this pastel set to get more girls interested in their products. I have no idea whether this marketing worked or not.
This set, which is officially known as Girls' Starter Set with Storage, has almost as many pieces as the earlier storage set.
Obviously, many boys will want to play with this set, just as many girls will like the others too. No need to pigeonhole anyone here.
The pieces in this pastel set are in the list below.
The Figure Assortment Set Gives You More Bits
I'm not 100% certain what you get in the Figure Assortment set (T0793), but it may include the following pieces.
Spin the Gears...You Might Even Learn Something
With the Trio Gears Set (V9195), Fisher-Price gave kids something really special to play (and perhaps learn) with. The picture above doesn't really do it justice, but it's a decent start.
The list below may give you a better idea of the possibilities. Notice that you get 13 gears in total.