Don't take a book. Nothing says, I don't hold out much hope for tonight than a selection of reading material.
Pack very light. You don't need more than a casual overnight bag.
Wear jeans/skirt, and take a top for Day 2
underwear x 2 (nice undie set for dinner; fresh pants for Day 2; maybe a spare pair in case of some weird emergency)
toilet bag (with condoms)
(pared-down) make up bag
nightwear only if you've of the 'what if there's a fire?' persuasion (in which case, there will be sheets/dressing gowns to wrap yourself in, both of which will make you look more intriguing/sexy to fellow guests than a stout pair of wincyette pjs)
And that's it. Get your hair done/do your hair and style out the next-day bed head look rather than carting half a salon's worth of dryer/shampoo/serum, etc with you. Two pairs of shoes max, unless hotel also has assault course and you think you might need trainers.
Maybe your bikini if there's a spa? I'm always torn on spas in hotel minibreaks. On the one hand, a massage is nice as is a dip in the pool, but on the other, if you've gone for minimal packing, there's a high Frizz Risk in wafting around in steamy atmospheres, and hotel dryers/mini shampoos are no one's friend.