I have an allotment and i absolutely love it.....it was in a dreadful state when I took in on in October last year (natch) but now I have fruit bushes planted and 4 x 13' raised beds.
The plan is to put in 3 more raised beds in the next few weeks, then membrane the rest til I'm ready to work it.
I'm not sure about the idea of grassing a bit over - you may well find that the site has rules against that sort of thing! Mine is quite relaxed, but others have rules about how much you need to have got under cultivation in the first year, and how long before it must all be cultivated ..... I'd definitely check this out before putting grass down.