Good idea!
But I do think it's worth remembering that there are people who could, and can, do a job like litter picking, but couldn't take on tasks needing more initiative - it's a valuable task, and shouldn't be looked down on. The brain injured chap who works with us has really benefitted from the chance to do something relatively simple and routine.
My national service idea is partly to break the link between college and uni - I suspect lots of kids go on to uni 'just because', rather than really thinking about what they want to do - focussed national service might give them a chance to try something and find out if it suits them, get them into the routine of a job etc. There would be allocations to all sorts of useful lines of work - environmental, care, classroom assistance - and perhaps a lottery for a certain number of places overseas, to allow those without funds to take a useful 'gap year' and travel. There'd be a basic supporting wage for people to live at home, and better help for those for whom home was no longer an option.
I'd even suggest that those with rich mummies and daddies could go off and have their gap years at their own expense - but they'd have to bear in mind that they needed to rack up experience and maturity to mathch that of those who stayed home and did a useful job and were therefore ahead in the job stakes.