I first modified the table by fitting a small vise. But I found the bench far too low and awkward to use. Despite this I have actually done a power of work on it at the low height of 67cm ( 2 foot 2 1/2 inches).
The height of the workbench in my small workshop is 94 cm ( 3 foot 1 inch). I was going to raise the outdoor workbench to this height but I then remembered a chapter I had read in L. Francis Herreshoffs' book The Compleat Cruiser (Note the American spelling of 'Complete'!). In Chapter 2 Herreshoff provides a nice diagram of an amateur workshop and discusses many things including bench heights.....
"............. I used to do the planing of planks on the long bench, which is 35 inches high, like most benches, but you can certainly plane easier on a table 29 1/2 inches high. It is the general custom to have the top of the vise right at the height of a man's elbow, believing that one can file straighter and evener at this height, but I prefer a vise one or two inches lower than this because the work itself is generally held above the vise".
By simply using timber I had in hand I have compromised between the height of my small workshop bench and Herroshoffs recommendations and obtained a good working height that fits my own body height and way of working.
So the comparisons now break down like this:
Workshop bench height = 37 inches (94 cm)
Original unmodified outdoor table height = 26.5 inches (67cm)
New outdoor workbench height = 32.5 inches (82.5 cm)
Herreshoffs recommendation = 29.5 inches (74.93 cm)
This issue of heights may seem to some to be a trivial thing but I can assure you that if you are putting in hours at a time at a workbench then it's imperative that you make sure that the working height is comfortable.
Further modifications will be to provide some diagonal bracing at either end of the table to make the whole outfit as rigid as possible and the addition of some plywood shelves to hold tools and lengths of wood etc.