I try to build walls and castles in my coastal cities, especially if I have stone. The protection and culture are nice, but the extra trade route is a big incentive....and it only expires when you get FM, which means an extra trade route anyway.
I haven't built many explorers, although the Explorer-Medic idea sounds like something I should consider.
I rarely build Hermitage unless I'm going for a cultural win or I have a city that needs to generate culture to push-back foreign culture.
Pre-BtS, I rarely built forts. Now, they rock!
You know something else I almost never build? Temple of Artemis. It just never seems to work out for me. Plus, I can never remember if the trade-route bonus is just for that city or across-the-board for all cities....