1. Interstellar travel: people don't realize just how large the energy budgets are to get to even the nearest stars, even with pure matter-to-energy conversion. This is, of course, the theoretical upper limit of efficiency but we have nothing remotely close to it. Chemical rockets are complete nonstarter becasue of the mass of the fuel makes the entire thing nonviable, even in a theoretical sense, beyond a travel time of hundreds of thousands of years. Even then you need energy to survive so it's unclear if you have enough.

So what do you do? Well, if you can reach interstellar speeds without using fuel you've solved so many problems. How do you do that? You focus energy from the Sun onto effectively a solar sail. You still need to slow down at the other end but you get some of this for "free" with resistance from the interstellar medium.

2. Computers. Our ability to utilize extra computing power shouldn't be underestimated. One possibility is virtual worlds. One estimate I've seen is that you need about 10^15 operations/second to simulate a human brain. A Matrioshka Brain (basically a Dyson Swarm that's essentially a giant computer) gets to (IIRC) ~10^80 operations/second. AIs that are basically people could live an entire virtual existence.

3. Weapons. Basically, if you have a Dyson Swarm you could sterilize the galaxy in about 100,000 years if you wanted to with a so-called Nicoll-Dyson Beam. Or use relativistic kill missiles taht are just basically lumps of metal or rock at near light speed.