A few key tools and considerations bear discussion.
Tape Measure. When it comes to marking and measuring and laying out, the tape measure has a thousand different uses. For your workshop, make sure that you have at least a three-quarter-inch-wide, ten- or twelve-foot-long tape. I find a small (six-foot-long, half-inch wide) pocket tape goes with me almost everywhere.
Try Square. A try square helps mark offcuts, identify what's square (and, equally important, what isn't), and belongs on your workbench at all times. There are numerous designs to choose from, but probably the most versatile is a combination square.
Torpedo Level. It's small, portable, and invaluable for a great many leveling-off or plumbing-up tasks about the place.
Bevel Gauge. Angle cuts, anyone? This is a tool that takes the guesswork out of matching an existing angle; it's inexpensive, easy to use, and useful indeed.
Find safe but accessible places in your shop for these tools:
The Hammer. Hammers have countless jobs to do in the workshop. I find that for the sort of finish work more often encountered in a workshop a fairly light hammer (perhaps fourteen or sixteen ounces) with a smooth belled face (it's slightly convex) is good. A wooden mallet is handy, too, for driving chisels, fitting workpieces, adjusting planes, and many other little tasks.
Chisels. A set of sturdy chisels will become invaluable to you over time. Good chisels are worth the added investment: They keep their edges and are safer to use (sharp tools require less pressure to drive and are less likely to break free when forced). On the other hand, top-of-the-line chisels are probably not required for the average Saturday-morning, let's-fix-the-broken-toy kind of workshop.
The Block Plane. For smoothing end grain, it's unsurpassed. For fitting and trim work, it fits right in a pocket. If you have only one plane, this is probably your best choice.
In the workshop, your needs must dictate the number and range of saws you have, but the more exacting work typically done in a workshop argues for more rather than fewer handsaws.
No matter how many highly engineered power saws I collect, there will always be a place for handsaws in my workshop. Perhaps we don't need quite the same range that were required before electricity or even steam power became commonplace, but I'd recommend a minimum of a good crosscut handsaw, a hacksaw, a small backsaw (like a dovetail or a gentleman's saw, or perhaps the Japanese equivalent, a dozuki), and a coping saw. But there are arguments for a good many more, too.