The Lost Boys is hardly a hidden gem. It was massive at the time and is still regarded as a classic.
A few I can think of are
D.A.R.R.Y.L.
A Sense Of Freedom - excellent film about Jimmy Boyle.
Society
Any of the "Popiscle" films.
It doesn't quite hold together when they get into space, but it's a great little movie and all three of the young leaRAB present themselves really well.
River is missed
I'd suggest Silver Bullet. It's a load of shit, but good fun all the same.
Walter Hill's very stylish and way ahead of its time movie with Willem Dafoe and Michael Pare. Some fantastic music, rain soaked neon coloured streets and a basic "bad guys kidnap singer, good guy goes to rescue her" storyline.
I also like the Peter Greenaway films from the 80s - The Falls, The Draughtsman's Contract, A Zed and Two Noughts, Belly of an Architect, Drowning by Numbers, The Cook the Thief His Wife and Her Lover.
There was a lovely film I saw one afternoon on telly when I was bunking off school. It was based on the French novel "E=mc2 mon amour" and was about a brainy American girl meeting a clever French boy in Paris and running away to Venice with him. It was really lovely (apart from the weird bit with Laurence Olivier, that wasn't in the book) but I can't remember what it was called. I know they didn't use the book title...