How far in advance to book? As you see from the web-site, lumbalumba is not a large place. I suggest you email them with proposed dates and ask what the availability situation is. By the way, they will do the transfers to-from the airport so no hassle there.
lumbalumba is nearer Manado than Tasik Ria. There is lots to do in the area. Taxis are inexpensive. Lumbalumba will arrange trips to the volcano, highlands - amazing scenery, warunga burials, the villages that build the Minnehasan houses, Tomohon the central town in the highlands with lots of horse-drawn buggies, ox-carts etc. These trips would be whole day or maybe half day. Tangkoko National park (see the tiny tarsia monkeys) is a long day or do it as an overnight away in a simple hostel. Taking a taxi into Manado (or get a lift with anyone going in from lumbalumba) is about a 30 minute ride. Taxis are either on the meter or agree price in advance. Manado is a noisy, bustling town with a ferry port and market, also modern shops and supermarkets, and places to eat. We have never had any problems walking around (any more than you would have anywhere eg London - please be sensible and not have wads of money sticking out of your pocket). Herself is perfectly comfortable and never been hassled. We would go in from lumbalumba from time to time and eat out some evenings. Roel and Juud will give advice on where to go.
Hope this helps.