Depends on what you're into/what part of the state you're in. There's tons of creeking to do in Southcentral, highlights include the Upper Upper Willow and Ingram Creek. Six Mile is the classic IV/V- run of the area, runs April-November, there's a bunch of creeks near it too. For big water, there's the Lion's Head section of the Matanuska, class IV, good cat/raft run. Also a bunch of higher-flow stuff down by Valdez and up in the Wrangell's, the Lowe (class III play run), the Tsaina (Class III-V), Kotsina (V), and Kuskulana (III/IV, in a gorgeous but very committing tight, sheer-walled gorge). Nenana's fun III/IV play/raft day run as well, great festival there every summer.
For more float-trippy stuff, there's a bunch of fun multi-day stretches of the Copper River, the Gulkana, and the Matanuska. The Talkeetna's a classic three-day III/IV raft trip, fly in, lots of bears. There's a few long stretches of the Yukon with fairly easy (no flying) logistics, easy to put together a 10-day canoe/raft trip, no permits. And then there's the whole Brooks Range for ultra-remote fly-ins.
But I digress. Favorite Creek: Upper Upper Willow Creek; Big Water Float: Yukon River (or Alsek/Tatshenshini?). Raft Fly-in: Talkeetna River.
There's two good AK guidebooks, the old-school Fast and Cold by Andrew Embick, not sure if it's in print any more but you can probably find a copy around somewhere, it's got everything from I-V, small and big rivers, some epic tales too. Then there's the new Alaska Whitewater and Creeks by Tim Johnson, focuses on creeks in Southcentral, but has a bunch of other runs as well.
Anyhow. Alaska's awesome, have fun up there. Drop me a PM if you want any specific river beta.