ive been going to lidl on and off for about 5 years, ive found their best things ive tried and still go back for are:
NIXE tuna salad (sold in tins for £1, American salad version is best) - almost as good as John west light lunches.
bbq pork ribs (~£2.50-3 in the chilled section near milk) fantastic with chips, not not usually keen on lidl meat but this is the exception, its marinaded in bbq sauce.
black forest smoked ham (again in chilled section), not the cheapest but not too expensive either and nice with croissants.
primadonna olive oil. came top in which? review, for £2.50 for 750ml its got quite a bit of taste for its price, not overly fruity flavoured so good on food.
emmental cheese, for £2 its ok considering the high price of president emmental chees sold everywhere else. ive tried the parmesan and wasnt blown away by it, just as tasteless as other supermarket parmesan.
other stuff: sweets, chocolate, ice creams, fruit when on half price offer.