The shares you pick for your ISA should be nice, safe, dividend-paying ones, shouldn’t they? Mostly, yes, I’d say. But what if you have a spare £500 to £1,000 that you can still stick in this year’s allowance just before it closes, you already have some safe investments stashed away, and you fancy a bit more risk (and hopefully reward) with it? Go ahead, I say, and I even have a few candidates you might want to consider.

Mining recovery

After a five-year share price slide of 57%, will Rio Tinto (LSE: RIO) recover? It surely will, and the only real question is when — but if you’re investing in an ISA for several years then the timing really shouldn’t bother you too much. The commodities price slump has led to a bit of a collapse in earnings per share (EPS) for the mining giant in the last couple of years, and there’s a further 50% drop forecast for the current year to December 2016.

But the prices of aluminium, copper, iron, gold are picking up again, and along with that should come an upturn in Rio’s fortunes. In fact, since 20 January, the share price has already gone up 26%, to 1,990p. And the City’s analysts have a recovery in EPS pencilled in for 2017 and they’re putting out a strong ‘buy’ consensus on the stock.

When Rio revealed full-year results in February, we heard that its progressive dividend policy is to be curtailed and that the payout should drop by nearly half in 2016. But that would still yield around 4.5%, and it is surely in the best long-term interests of shareholders as the firm continues to cut its costs. Yes, I see Rio as an attractive ISA buy now.

Minerals punt

There’s something a good bit riskier, but possibly very lucrative, in the shape of Sirius Minerals (LSE: SXX), the company at the heart of the York Potash Project. The shares tend to spike up on good news before falling back again, but they’re up 55% over 12 months to 14.7p today, and could well have a lot further to go.

On 17 March we saw the Definitive Feasibility Study, which valued its polyhalite potash development at a net present value of $15bn, rising to $27bn by the time production commences (although that’s not expected until 2021). With the deposits being of such high quality, and with fertilizer in such great demand, it should be a high-margin business too, with cash margins estimated at 70-85%.

So why is Sirius only valued at around £300m? The thing is, oodles of cash will be needed for developing the project before any profits can be made, and there’s a great deal of uncertainty over how much will find its way into the pockets of those who provide the capital and how much will be left for existing shareholders. But if you don’t mind the risk, I think things are looking good.

Too late?

You might think I’m a day late introducing Intelligent Energy (LSE: IEH) as a last-minute ISA candidate, with its shares having spiked up 65% on the day to 16p. But what’s happened? The company develops fuel cell technology, and there are sure to be some big profits for the winners in the field, and today we heard news that should boost Intelligent Energy’s prospects of getting to that magic profitability point.

The company is still in discussions to secure the funding it needs, and though there’s no certainty they will conclude successfully, it seems optimistic, saying it hopes to finalize something shortly. Perhaps more importantly, the board intends to restructure the company to focus on the best market opportunities, which apparently means mainly its “class leading and power dense sub 1W to 20kW air cooled fuel cell technologies which are targeted towards small to medium-sized, highly distributed applications to power a range of off-grid devices“.

Analysts are forecasting first profits in 2017, making Intelligent Energy definitely one to watch.

Including a few growth candidates in your ISA can be a profitable strategy if you plan it carefully, don't take needless risks, and diversify your picks. That's why I recommend you get a copy of our latest free report, A Top Growth Share From The Motley Fool.

It's not a high-risk tiddler. In fact, it has a market cap of around £1.5bn, very little debt, and the Fool's top analysts think there could be handsome rewards for those who invest now.

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Alan Oscroft has no position in any shares mentioned. The Motley Fool UK has recommended Rio Tinto. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.