The Transition to Algebra (TTA) project, an initiative of the Learning and Teaching Division at Education Development Center, Inc. (EDC) includes a wonderful collection of Mobile Puzzles. Visit solveme.edc.org to play SolveMe Mobiles (also available for the iPad.)
Looking at the menu, you will see categories with different levels of difficulty available from very simple puzzles to rather more complex puzzles which promote good mathematical thinking.
Students must determine the weight of each object shown which makes a good introduction to the skills required to solve equations, linear and simultaneous.
Selecting ‘Information’ provides extensive help; note that various tools are available so you can annotate puzzles and / or add symbols and equations.
Note that you can then drag a heart to subtract a heart from both sides:
Note that under settings you can choose to show numbers in the mobile as in the illustration. If the solution is correct, the mobile will balance.
On the other hand….