I considered bringing one of our Lego Simple Machines kits. They're educational, but expensive. If one of those parts got lost enroute, replacing it would be a frustration (especially since I can't FIND the kits on-line anymore).
So, I improvised.
I found a see-through green 8 inch x 4 inch x 1 inch plastic box with 5 sections and an attached, latching lid. The PERFECT SIZE to slip into a backpack! It used to hold fishing lures from a kid's fishing pole kit we got YEARS ago. It's been bouncing around the house every since, used for holding beads, hair pretties, and STUFF.
Searching through the Lego totes, I found flat pieces in gray, green and blue. I used these pieces to make a road and parking area (gray), grassy places (green) and a lake (blue) on top of the lid. Hot Glue worked awesome to attach the pieces! I did have to cut (with kitchen scissors) on long green piece to fit 2 sections.
FYI: Make sure the glue does not spread out beyond the edges of each piece as you want them to fit SNUGLY next to one another.
I then searched for a "car chassis", a Lego piece with wheels, and specialized pieces that I thought Sam would like to build a vehicle. I included levers, windows, a wheel and boat rudder (and he could make a boat for the water too), red and clear single "bump" Legos (for headlights), antennas, and 2 Mini-Figures, and then threw in a number of varies sized bricks. All these I fit inside the box. And for now they're organized!
FYI: Don't forget a Mini-Figure or two for more imaginative play.
Voila! A Travel Lego Box with things we had lying around the house. And if a piece goes missing? No biggie! (No laughing at my QUICK build to try out the box's top! LOL And yes, that red-and-yellow THING in the lake IS a sailboat...)