A work bench top that's 3 or 4 inches thick can provide you mass, which will ensure the entire bench is sturdy. There are also special work benches that offer sturdiness through their designs like mortise and tenons, and splayed legs. They are the ones that let you have a workspace that doesn't have a massive slab top.
There's no other way to go about it; your garage work bench must be able to deliver stability and sturdiness. Sawing and hand planing on cheap, flimsy workbenches can easily become frustrating, not to mention dangerous, as they'll be moving and shaking across the room. A sturdy garage work bench should have a solid construction and good joinery.