I've stayed in the old city and thought it was a little crowded and busy. I've stayed near the river and found it a bit boring.
What's the area just north of the Princess Mothers Health Garden and east of the uni like? I can see a number of hotels are located there.
Pingnakorn Hotel on Nimmanhemin rd soi 12 gets good reviews - what's that soi like?
I live just next to Nimman Soi 3 and I love the area.
I don't know anything about Pingnakorn hotel though.
Nimman is a great area, lots of decent bars and coffee shops - Ristr8o a must try - but not many places to meet other travellers in this area as it's a bit out of their price range and mainly a Thai area.
If you're staying in CM for a while and don't mind paying slightly more, I'd recommend staying around Nimman.