You should increase the returns you make on this capital. Buy some books, take some courses, invest in your health, hire someone to design your portfolio. If you get a job with 10% more payment, you effectively doubled your investment after one year, tripled id after two and so on.
Its lame, I know. Alternatively start and finish a side-project and invest the money in PPC ads.
In that case you could pick up some books on investing in shares. This has good liquidity and although risky it is unlikely you will lose 100%. If you have a portfolio of shares in different sectors, you should be at low risk of losing lots.
If you fancy learning but still a passive investment, then maybe stamps, antiques, collectibles. You will get additional enjoyment out of it in the meantime.
Some parts of the UK I imagine you could use 5k for a house down-payment and rent it out. I have been expat for 4 years so I have no idea.
If you want to be more involved you could start some kind of business, using 50-100 pounds and your own effort to get it going, then 'invest' more money once you have a proven method. For example once you can spend 100 on advertising to make 200 gross profit, then you can scale that up using the 5000, then keep reinvesting to grow it. Easier said than done!
Or you could renovate something. Lots of people have broken things they would give away or sell cheaply - you repair and sell on for a profit. But you will need some capital and that is how you can invest your 5000.
Maybe in Merthyr, but even 4 years ago you'd have been lucky to find a house under £100K without anything unrentably wrong with it.
I can get a 2 bedroom apartment in the city centre in Liverpool for <£100k and it would rent out for £600/month easily.
However, Looking on Rightmove, I see no houses in L1 for that price, but I do concede that there are plenty near the L1 edges of L3, L7 and L8.
For what it's worth, it's probably not a bad idea to put it in the S&P 500[2]. In the long run, it's probably going to outperform most other investment strategies, although if the US economy tanks in the next few years you could lose a good bit of what you put in there.
I also think that allendoerfer's advice about investing in yourself can be very good, depending on your situation
[1]http://personalfinance.duke.edu/prepare-your-future/savings-...
[2]https://personal.vanguard.com/us/funds/snapshot?FundId=0040&...
This comment should not be construed as a replacement for professional financial investment advice.
Secondly, just holding the nickels doesn't mean you're sure to not lose any money. As you said, simply stuffing cash (or nickels) under your mattress means you're losing some every day to inflation. You're also losing all the potential gains of the other things you could be doing with that money other than piling up nickels. At the very least, you take no risk of losing the principal by sticking it in a savings account (~1% APY) or buying a 1-5 year CD (2-3%).
Then again, I don't have a couple million dollars to blow on stockpiling nickels. The sage investor must know something I don't.
1: http://www.coinflation.com/coins/1946-2007-Jefferson-Nickel-...
ETFs (Exchange Traded Funds) and Investment Trusts are similar to investment funds in that they are also collectives, relatively low cost. The current fashion in investing is to buy 'passive', 'tracker' or 'index' funds (or ETFs), which all follow an index such as the FTSE or S&P. These tend to be lower cost than 'actively managed' funds (where the fund manager tries to beat the index). Vanguard is a popular 'passive' fund manager.
Other types of investments include property (you generally need quite a lot of capital), and more risky types of investment such as forex trading, spread betting, etc. Even experienced investors tend to consider these risky.
Investments have done reasonably well over the past year in some areas (some UK income funds are up 8% in the past 12 months), not so well in others (a FTSE All Share tracker is only up 0.6% over the year). Investing is not risk-free, and generally speaking there is no guarantee to make a profit. You could also consider saving in cash, but with interest rates as they are now, you'd probably be worse off in real terms by the end of the year than you were at the beginning (because interest rates are generally lower than inflation)
Some investment brokers in the UK include: rplan (disclosure: I work for rplan), Hargreaves Lansdown (the largest), Fidelity, Nutmeg (another startup).
As others have mentioned, it's definitely worth doing a bit of research to find out more about how things work. If you don't mind the shameless plug, we have a 'guide to investing' available on our site which we think is quite a good overview of what's available (it's fairly UK-specific though): https://www.rplan.co.uk/investment-guide (note: registration required, let me know if you'd rather not register and I can send you a copy).