You can reach Bukavu from Rwanda via the borders of Cangungu, from Burundi via the border of Uvira, and from Goma via Lake Kivu or by Road. Due to unresolved wars that continue to this day, as well as the massive crossing of goods over the Rwanda/DRC border, the border crossings can take a great deal of time, as well bribes. Do not approach expecting to be able to buy a visa at the border. Have it beforehand and also be prepared to wait and still possibly pay a bribe. It's just one fellow writing everything down in a big ledger.
At the time of writing the road to Goma[ is not considered safe but there are boats every day. Expect to pay $10 - $50 depending on speed / comfort. If you're white you may have to argue to get a cheap ticket, apparently it's 'not suitable for tourists'. You may have to pay $5 departure tax - they gave me a receipt so I suspect it's official but you never know here. Occasionally these boats also stop at Idjwi Island
There are wooden boats to Idjwi island leaving at various times through the day. Ask around at the port. Unlike the northern part of the lake the water here is generally calmer (due to it being narrow and populated with a high number of islands) so the voyage by wooden boat is not usually unduly traumatic.
Bukavu is mainly a dollar economy, so plan to use Congolese francs only for small purchases. For shopping, check out the SuperMarket Asante, between the Mayor building and SONAs, in front of the Bukavu Cathedral.
For lunch and dinner, try the Cafe and Restaurant Asante at Supermarket Asante, located between the Mayor Building and SONAS.
If you want to get out Bukavu City: Please visit Kaziba about 35 mileages (55km) from the big city Bukavu. There is a guest House at Byakweli's property, closer to the Kaziba Hospital.