What's wrong with the Subaru? Are you saying you want something with more room? If so, the CRV and Rav4 are roughly comparable or a little bit smaller than the Subaru. The Highlander probably would be good, but the trick is finding one with the more fuel-economy friendly options. With the 4-cylinder engine and only front wheel drive the highlander is a pretty frugal little vehicle. With the V6, it's a real gas hog and the hybrid drive train, though good in town, gets even worse mileage than the V6 on the highway! The trouble is that they're sold as a luxury car most places, so you'll mostly find V6's and Hybrids in the used market and even on most new car dealer's lots-- you may have to do some work to find a base-model one.
Of course, and I have to say this every "need a fuel efficient big car" thread, but what you really need is a full-size station wagon, but you haven't been able to buy one of those since 1996.