It's a quiet city with a nice feel to it. Not much happens unless you make it happen. It's a good place to practice your French too. The small tourist area next to the Old Market (where you can buy lots of fruit) has a good selection of restaurants and bars.
If your time is limited, don't feel bad about missing out Battambang (pronounced Battambong) although I liked it a lot.
Accommodation
Seng Hout Hotel
One of the more popular hotels, right next to the Old Market and only a few minutes from Pub Street. There's a tiny swimming pool and a rooftop bar. I paid $15 for a good room with A/C.
Food
Jaan Bai
Very popular cafe/restaurant. They have a tasting menu that looked good but minimum of 2 people (poor me). I had the Pad Thai and crab. Both were good but the crab was hard to eat without getting very messy.
Nary Kitchen
See cooking class below.
Flavors of India
Pretty decent Indian food. Probably not worth it if you're going to be back in the UK soon!
BTB Pizza
Same deal as above. Not worth it unless you have a craving for pizza. It was very tasty though!
Nightlife
Pub Street
There are lots of little bars around the Pub Street area. It's not crazy at all like the Siem Reap namesake.
Sights & Activities
Soksabike
A great social enterprise bike tour of local industries. We visited homes of people making rice paper, rice wine, dried bananas and other bits. You get sweaty but it's worth it. Recommended!
Nary Kitchen cooking class
A fun cooking class near the Old Market. They have a morning and afternoon class. You eat quite a lot of food at the end of the class so the afternoon class probably makes more sense.
Boat ride to Siem Reap
If you need to get from Battambang to Siem Reap or vice-versa, you can take a bus which gets you there fairly quickly, or you can choose the scenic route and go via boat. I'd had this boat trip recommended to me by a local very early on during my stay in Cambodia. I'd then read lots of positive things about it. If time isn't an issue for you, then I recommend it. It took 8 hours and I wish it'd been half that. Really interesting seeing all the fishing villages along the river.