If you’ve been working or studying the rest of the year, there’s nothing wrong at all in taking something of a vacation during the summer. Quite the contrary, a decent break is a necessity, which is why many countries have mandatory amounts of annual leave for workers.
Perhaps you can find some volunteering to do - charity work, or helping coach sports if sport is your thing - if the idea of doing nothing is unappealing? Or if study is more your style, start seriously studying your faith - read the Catechism or the Compendium of the Catechism, scripture or other books about the faith. There’s a lot of stuff available online, or you can check out what your local library has to offer.
That should keep the lazies at bay!
As for your second question - it’s a gray area.
Lust is always a mortal sin, as the Gospels demonstrate.
Otherwise it depends on how much you indulge, partly. For example overeating on occasion is venial sin at most, overeating most days is probably grave. Spending the occasional day doing nothing useful is fine, spending weeks on end (unless you’re on vacation

), especially when there’s study or work that you should be doing, or jobs you should be applying for, would be grave/mortal.
Probably also depends on how much your vice interferes with you doing the things you know you should - be they study or work or prayer. I know at times I spend way too much time with the TV or the internet and not enough on the other things - that’s when I know I’ve got to hit that confessional.