Yeah that hotel is right around where a bunch of the "Bricktown" hotels are. It is basically a truck stop and there aren't the best crowds that hang around, but if you are on your own I am sure it will be fine. There is a Waffle House down there and the Greyhound bus station is moving to that intersection because they didn't want "those people" downtown anymore. Very cool they have a free shuttle!
As to the Plaza District, it isn't very big, so don't plan on being able to spend more than a couple hours there before you see it all. It is basically the 1700 block of NW 16th ST, 2 miles from the Amtrak station, 5 miles from that hotel.
The Harkins theater in Bricktown is just a plain old movie theater built a few years ago, nothing special about it. There is a place called Red Pin bowling right next to it, interesting bar / bowling atmosphere. There was a great Jazz club that just got shut down for tax evasion, but if you like live music, Jazmo's Bourbon Street Cafe and the Biting Sow should have some.
If your a drinker, be aware that all domestic beers (bud, coors, miller, and every other brand/variation they sell) sold in the entire state are 3.2% instead of the normal 5%. So best to stick with micro-brews and imports if you don't like watered down tasting beers. Always good to double check with the server though, as I have been amazed at what brands will make a 3.2% version (Heineken, Corona, even Sam Adams). Also any advertised happy hour special will always be 3.2% beer, state law.
I have not been on a river cruise, but plan to do so when we have some family visit.
Almost forgot to mention Midtown (Walker & NW 10th ST). Great group of independent restaurants / bars, it is walking distance from Bricktown if your feeling up for one (1.3 miles and the Overholser Mansion is just a few blocks north of there on NW 15th).
Our favorite breakfast there is Cafe Do Brazil, on Sat & Sun from 10-2 they do brunch. Everything is under $10 and is really good, they make their own re-fried black beans and there are $2 Mimosas and $3 bloody marys.
http://www.cafedobrazilokc.com/
For lunch or dinner there is a great modern Italian place, they don't have a freezer or a microwave, everything is fresh. Never been disappointed.
http://www.stellaokc.com
Feel free to keep asking questions, I have only been here a little over a year but have done a lot of exploring in the city.