Jakarta airport money changers are OK but the best rates in your trip should be money changers Manado downtown.Don't change abroad, they will charge you fees or crappy rate.
There should be either an ATM or a money changer open at Jakarta airport in order to change a small amount to survive until 6AM. After then change in Manado (but not at the airport).
Considering 2 days trip in Manado I would keep 15USD each day to take a taxi to Manado and back to the hotel, 35USD for the dinner at Raja Sate for 2 persons and 10 USD miscellaneous (buy a Klapper taart at Cela Bakery to bring back to your room and enjoy with a coffee, etc.). For 2 days that would be 100-120USD, then in R4 try to stay nearby Royal Mamberamo there's an excellent carving and masks stall 400m walk up the road, Kotekas (Penis sheath) always make a good present they are sold around 10USD if you take some. Depending on what you expect to bring home count anything from 20USD to 100USD. There will be no restaurants worth outside your hotel so you just need too consider taxis to drive to the airport or elsewhere say 20USD.
All in all I would say 200 to 250USD equivalent in cash change (3Mils IDR) will be largely enough to live "la vida loca" for your 3 days, including souvenirs and eating out. If you still have some pocket money to spend on the way back buy Toraja or Papua coffee packs at the airport (NOT Luwak coffee which is a real scam), you'll be surprized how delicious they are.
As already said take two crisp bills of 100USD rather than a pocketful of 20USD notes which are worth not that much in Indonesia.