I really like simple circuit labs that explore the basic concepts of what electric circuits do. I know a lot of teachers want students to get into groovy, sophisticated circuits, but I prefer to spend time on the basics. And I have yet to encounter many students who find my elementary labs to be without challenge. Electric circuits are non-intuitive; students don't know what it's like to be an electron compelled to move through a circuit. So I'm a strong advocate of keeping it simple.
There are many things I do to that end. One is to engage my students in a number of "batteries and bulbs" activities. I like C- and D-cells, miniature screw-base bulbs and sockets, and connecting wires with alligator clips. And I like magnetic battery connectors. These gems hold batteries in series or in parallel. And they're easy to connect and disconnect.
I bought a set years ago from a now-defunct catalog. You had to buy them in packs of ten for about $55. Not cheap. But so useful in the lab their cost was easy to justify.
We use them in Physics 1 for PhyzLabs Batteries and Bulbs, An Open and Short Case, and Electric Magnetism. In AP Physics 2, we use them in our RC circuit labs.
It bothered me that such a useful item was no longer available, so I always nagged scientific supply company reps at AAPT meetings.
Arbor Scientific's Peter Rea saw the value in the item. He asked me to send him one and he'd send me ten in return. It took a while, so he sent me twelve when he made the product available. Arbor calls them Magnetic Terminals and sells them for $1.25 each. I recommend four for each lab group.
Once you use them in your hands-on labs involving otherwise "loose" batteries, you won't want to be without them.