Here are my own two cents. First, a few general comments, in random order:
1. Definition of 'intelligent'
The crucial element is how you define 'intelligent'. I am not familiar with work on the subject, but at least in ordinary life 'intelligent' does not equate with 'good at something specific'. For example, one can be intelligent and good at math, or good at literature.
Would you call an algorithm, however complex, 'intelligent'? Such as the dating sites example somebody mentioned in another answer? I would not. My own imprecise definition would most likely focus on intelligent being related to perform in circumstances outside the design parameters... I wouldn't call any algorithm intelligent, regardless how well it does the job. Efficient, clever, nifty, yes, but not intelligent.
Two more dimensions of intelligence:
(1) There's a difference between being intelligent enough to recognize a pattern in the data stream (something where computers are getting quite good at), and being intelligent enough to formulate a theory as to why it happens so (i.e. understand and therefore be able to predict the future).
(2) There seems a lot of confusion between 'intelligent' and 'possessing free will' and being able to act on one's own initiative. These are not identical.
2. Definition of 'good at', or 'better'
This requires some external objective criterion to work. You should be aware that there are some things that are quantifiable (and can therefore be compared to each other and ranked), and there are things that are results of value judgments (moral values, or preferences). The latter cannot be compared to each other, they are equally valid.
Also, these usually do not translate well when used in the context of performing a concrete task. For that, you need 'adequate' (can do the job) and 'cost' (how much it will cost you in terms of some limited resource). To be efficient you pick the cheapest way that does the job, not the absolute best. Look up 'absolute advantage' and 'comparative advantage' in economics. In layman's terms, you should not ask a genius to perform a routine task, assign the really difficult tasks to him/her.
3. Ability to understand & modify oneself
@Christopher Creutzig already alluded to something similar. An AI might (should?) be able to understand how it works, at least it should be easy enough to upload the design specs to it. And, more importantly, should be able to modify its internal workings to better align itself to the task at hand. In a sense, should be much better at focusing on whatever it should be doing and not being distracted.
This also includes ability to recognize insufficient ability and being able to load skills modules as needed ("I know kung-fu."). In an AI setting this would be near instantaneous, while in humans it is not. This is both a boon and a bane. A boon because it is available immediately, and is a 100% copy of the original skill. A bane because of the same things -- human learning also is closely related to introducing variations in the skill being learned, which ultimately may result in its improvement.
I am not aware of any two humans that are 100% alike, or were 100% alike at any point in time. AIs are by definition, at least given current technology, a digital set of records and as such can be replicated at will. Of course, as they gain experience, they will diverge, but we at least have the possibility to have them being 'born' absolutely identical. Unless of course getting to a true AI requires getting so into quantum stuff that you cannot get two identical copies.
There are two corollaries of this:
(1) The ability to copy & reproduce, i.e. spawn multiple exact copies -- certainly not feasible for humans today. Very useful for anything that requires multiple autonomous but very similar units (the army?).
(2) Moving to a more powerful hardware platform. Think over-clocking the CPU, or moving from a laptop to a mainframe. You can run the same 'program' on a vastly faster hardware.
5. Memory vs thought process
Most definitions of AI seem to relate to the thought process, i.e. how well it mimics the human thought process. One major advantage (or possibly disadvantage too!) of a computer/AI is the ability to quickly access information, incl. one's past experiences. Think perfect recall (a direct record of all sensor inputs, etc.) No human I know can do that.
Finally, on your points:
Computers are already substantially better than humans in many repetitive tasks, such as simple(*) computation. The advantage is vastly increased when parallel computation can be employed. Similarly, an AI should be able to simultaneously do many things (relatively simple to its level of 'intelligence' or computing power, of course).
This translates directly into better inventory management -- keeping track of whatever 'resources' you have. In this sense computers (yes, no need for AI) should already be better than humans at long-term planning if the rules of the game do not change. If they do, esp. in an unpredictable ways, humans seem better at adjusting the goals and the definitions.
(*) "Simple" means anything that follows a pre-written program, however complex. Not simple begins when the AI is able to select both the algorithms and the criteria itself. I do not know enough whether we're there yet.
How could I make a system where the humans and AI are both equally relevant in accomplishing the task?
Aside from the obvious -- choose an appropriate task where they are equally relevant despite the differences, due to some external factor, see below for example -- there are two possibilities, not mutually exclusive:
(1) Take away the clear advantages discussed above. That is, simply substitute the human biological 'hardware' with an equivalent machine. Like Asimov's positronic brain (from memory, they're all stamped from the same production matrix but due to some quantum stuff they come out different).
(2) Allow humans to get some of the advantages (e.g. implants, ability to be scanned into a machine and remain a 100% percent copy, etc.)
These might lead to a regression to the mean, i.e. blurring the difference between man and machine. If a human can be scanned into a machine -- is this an AI or human?
What situations could help AIs and humans be equal -- there a some ideas in the other answers. Think limited energy budgets for example. Put an economics context -- you need to take into account both the benefits and the costs. E.g. an AI might be much better at something, but still be prohibitively expensive. A simple situation -- you're on the road, well away from civilization. You need to solve a simple problem -- add 50 4-digit numbers. You could drive back home and use your computer, or you could use a pen an paper... The computer is still vastly superior but pen & paper are both cheaper and adequate to the task at hand.