Buses leave from Addis Ababa's Kality bus station (at the south of the city) regularly. It is 10 km from the downtown (1/5 of the distance to Debre Zeyit), inconvenient to reach by public transportation and expensive by taxi, but there likely may be services from other, more conveniently located bus terminals around the city - ask locally, e.g. in the place where you're staying in Addis, about the best option from that location.
There are at least five small lakes in Debre Zeyit. Bishoftu Lake is the closest to the main road and has a few resorts located on its rim, with good views of the lake.
The main road has many hotels. On Bishoftu Lake's rim there are a couple of resorts.