If you want to take a shower after your long flight there is a place called Freshen Up at SFO.
From flysfo.com:
"Shower facilities are available at Freshen Up! located on the Departures/Ticketing Level of the International Terminal, near the entrance to Gates G91-G102. Freshen Up! is open daily from 7:00 a.m. to 11:00 p.m. For additional information call 650.877.0798."
If you're not tired I would also recommend hopping on BART downtown. The ride is only about 30 mins or so and the trains run every 15 mins. If you like shopping get off at Powell and there's tons of it. Plenty of other things you can see in the 2-3 hours you'll have to spend down there.
But it'll be getting late London time around then and you might be tired. If so, for that long of a layover I'd pay the $35 to use the UA RCC. The pass is only good for one RCC so you won't be able to hop between them... your Hawaii flight will leave from the domestic terminal but I'd probably choose the interntional RCC to lounge in for the day. Unfortunately there are no beds or places to lay down.
Another option... you appear to be on the last nonstop of the day to HNL. UA has another nonstop (on most days anyway) leaving at ~4:20pm. You could pay $50 and go standby on the earlier flight.. if you do just be sure you tell the agent at checkin you're doing standby so they can correctly tag your bag.