One has to ask - Where are you leaving the bike, and for how long? If it is going to be a 'couple of minutes' deal, I just use my 3/8" cable lock. For an 15 minutes- to an hour hour - a U-lock around frame and rear wheel that also utilizes the cable to secure the front wheel. My bikes are not out of my sight for longer than that!

Fortunately, my bikes are old enough (30-40 years old) that they're not hot items. Not flashy and usually dusty/dirty too! Not very valuable other than to me for sentimental reasons - say $250 tops for any of them. Typically a thief won't risk jail time for something worth so little money.

Seat? I've never had a problem with that (remember - it is rarely out of my sight for anything over 10 minutes) If you're that worried, use a tamper-resistant Torx bolt (looks like a round-head Carriage bolt, but has TR Torx socket) and a rounded-off nut that must be held with vice-grips or better yet a threaded seatpost.