Get a copy of Dawn Rooney's book, Angkor: A Guide to Cambodia's Wondrous Temples.

I might go out to Banteay Srei your first afternoon. I expect you will visit the temples close to Angkor Wat on your second day after sunrise there. There are many dozens of accessible temples, and you will only have time to see a few. So studying Dawn Rooney's book will help you decide which temples you'd especially like to see.

You can start touring temples that afternoon. The normal temple tours usually involve touring in the morning, then you go back to your hotel for lunch and a rest, and continue touring from about 3 to sunset. (Be kind...your guide and driver count on the mid-day break to go home. They won't be rude and say "no" to continuing, but will miss their big meal of the day) Hiring a guide for just the afternoon will be very easy.

You could also do non-temple things (visit the Old Market, local temples, bicycle around town, or visit Frangipani Spa for a massage, or go get your temple passes for the next day when they go on sale at 5 PM. It will save you time in the morning, and they are valid for the remainder of that day too. You could tour at least one temple before it gets dark. If you don't want to actually go into temples that evening, just have a tuk tuk (maybe $6 for an hour or so) drive you around the complex to get an idea of the grand layout. You can't drive around without a pass, even if you don't plan to stop.

i agree and kathie's idea is an excellent one as many love that small temple the most..

don't waste time in town or at the lake on day 1

If you are staying out for the day at least offer to buy your guide and driver lunch. If they take you to a restaurant out at the temples they will be probably be given a free meal in exchange for bringing you. My usual tuk tuk driver sometimes eats with me (if I don't have a guide) and often comes out to dinner with my daughter and myself.

You can look round markets and have massages in the evening so go templing during the day. You'll go past other temples on the road to Banteay Srei and if any of them look interesting stop on the way back.

Bookmarking

One and a half days is not much time especially when you have a full day planned for Angkor Watt. Banteay Srei is my favourite temple too but a long way out, you go through some villages and see the ladies making palm sugar on the side of the road and few other interesting sights, but you might be a bit too tired on your first afternoon and be better off just enjoying a relaxed lunch in the Pub Street area, then visiting the old market, maybe have a Dr. Fish foot spa just for a giggle.
Another option is to go out to one of the floating villages, there are three to chose from, it is nice to be out on the water at sunset so I usually go out to the villages about 3pm, I have a super local guide I usually use so can recomend if you need. Good news is SR is very safe and Pub Street is great for your evening meal, you can generally eat anywhere without worrying about the food or the safety, so enjoy.

Bookmarking. Thanks for the info!

Sharich,

My daughter and I will be visiting Siem Reap in February 2013. I'd be interested in knowing how to contact your guide. Thank you.

Sharich

I will be in SR in March and would also love the name and contact info for your guide!

I'm not Sharich, but here are some excellent guides that I have both toured with and know personally:
http://www.sevencandlesguesthouse.com/#!sample-itineraries/vstc2=our-guides