Just come back - and as always there are problems with exchanging US$.
The notes must be high value clean, new and preferably unfolded. Moneychangers offer better rates than the hotels - but will look at every single note, the slightest mark - they will not take it. They also will not touch some notes with some serial numbers.
However - they do seem to be better disposed towards euros - preferably clean and new of course.
Ride into Manado from Tasik Ria is less than an hour and taxis have meters and have clear blue and white paint style and clear labelling, and generally quite new. There are loads of them in Manado. For Tasik Ria they may negotiate a rate - between 40,000 and 50,000 rupiah is fine (I think that's about 4 - 5 dollars to you).
Email ecodivers at Tasik Ria - they will give you good advice about currency. Assuming you are diving with them you may need to know how they would prefer to be paid. Sandra at Ecodivers will reply promptly - if you get no reply, please re-send because the telecomms connections into Manado can drop out and they don't always receive their mail.