The Riverplace is very close to the Portland Streetcar line, and since you asked for really great restaurants, I suggest you take the Streetcar north to NW 21st St. to find 2 of Portland's finest restaurants just adjacent to the Streetcar stop. Both Wildwood and Paley's Place offer a sample of the finest in Northwest cuisine.
1. Walk 0.39 mile south from Riverplace Hotel (1510 SW Harbor Way) to SW River & Moody
2. 12:13 p.m. Board Portland Streetcar to NW 23rd Ave
3. 12:41 p.m. Get off at NW Northrup & 21st St
Aside from Murata, Carafe, and VQ (all good choices) there are a couple of good downscale choices close by.
There's Hot Pot City (their website is down so here is citysearch, it links a map):
And there is Tom's First Avenue Bento (they don't have a website at all):
Both are good for fresh and inexpensive lunches and Hot Pot City is also open for dinner (and they have a lot more seafood out for dinner). Both have about zero atmosphere and are kinda holes in the wall, but the food is good. Tom goes to the coast every few days to get fresh seafood so the fish and seafood choices are always great.
How close is close?
Within an easy walk are Murata and Carafe. Murata is one of the top couple places in town for sushi. Carafe is one of the better French bistros and still relatively casual. Veritable Quandry is also an easy walk away and provides a more upscale meal than these and has a nice patio. These are the closest places I can recommend, but if you add a 10 to 15 minute walk you open up most of downtown.