In reply to Adrien:
For a small taste of the walking on offer you could start with our UKH Route Cards:
http://www.ukhillwalking.com/logbook/r/find.php
They tend to be all-day routes, more than 5 hours certainly, though many can easily be shortened. And there are a few smaller ones in there too. The selection is far from comprehensive - you can walk pretty much anywhere, just open a map and go.
As others have said, wild camping is legal if done away from houses and if you do so responsibly (no litter, no fires, bury poo, move on after a couple of nights max...). There is a lifetime's worth of good spots to camp, of varying degrees of mushiness.
Also, to re-affirm what others have said, crowded hills are the exception in the highlands rather than the norm. Away form Glen Coe/Fort William this becomes increasingly the case. The midges are more likely to bother you than the crowds. Bring head nets - utterly essential for wild camping in summer.
Every bit of the coast form Ardgour (already mentioned by others) northwards to Cape Wrath is worth visiting, as are the islands (Mull is generally less crowded than Skye) You really can't go wrong wherever you end up. but a few area highlights would include:
Ardgour/Ardnamurchan
Mull
Skye
Rum and Eigg
Knoydart
Loch hourn (if you care to spend the time getting there)
Glen Shiel
Applecross
Torridon
Gairloch to Ullapool
Assynt
Cape Wrath-Durness-Tongue
It's a shame you only have two weeks