Most power adders will help at cruise. The big ones are Cold air/Fenderwell intake and high flow catalytic converter. Also porting and polishing goes a long ways, including intake runners, touching up the heads, and some work on the exhaust manifolds as well.
But, do them because you want power, not fuel mileage. I have done a couple odds and ends on mine, and have trouble breaking 20mpg on average, with about half my commute on the highway. I haven't been able to figure out why, but i suspect my heavy foot doesn't help any. That is the down side to modding, it helps it suck fuel that much quicker if you want it to. 2 edge sword really, half the time it helps, half the time it hurts.