Romlan is very nice at Tuk Tuk Samosir Island.
Here are my photos from 2006
Lake Toba has always had great accomodation for great prices.
And some reviews from Trip Advisor on Romlan.
Toba has so many varied options and none of them ever really fill up, that there's no point in booking ahead. I'd recommend staying at one for a night or two (Romlan, suggested by the other commenter, is a perfectly fine choice) and spending a few hours walking around and checking out other places.
That way you can find a place that's perfectly suited to your tastes. This worked well for me, because some of the most-recommended places didn't actually appeal to me at all.