I haven't done much specific wildlife photography in Yellowstone, but Lamar and Hayden valleys are the obvious places. I would definitely try to talk with someone in the know when you get there. Sometimes the Rangers are useful, but I have found most people standing around with very large lenses tend to be quite approachable and offer more current information. Especially early in the season. Of course you will need to be out there at dawn and dusk.
I have already seen a number of pictures of bears this year, but as with the wolves and other wildlife they tend to move around from week to week. 400mm is not long for wildlife, but it is possible to get quite close to bears and sometimes wolves to the point where it will be enough on a crop. It shouldn't deter you from trying. if your longest lens was 200mm I would suggest a good pair of binoculars instead!
As far as landscapes go, I'm a wide-angle nut, so 18mm on a crop would not really be wide enough for me, but I think it is still good for a lot of general landscapes/geysers.
I think the tripod idea is good. Personally I now have a pretty expensive light rig, but even a cheap light tripod (I guess I consider cheap being ~$150 - $200 for tripod and head) will be a lot better than nothing. As with cameras, the best tripod it the one you have with you. funny you mention rivers - not many people get into the rivers with their tripods, I know you said you wouldn't, but it's a good way to get something a bit different. You are often not allowed to be on the riverbank where there are geothermal features, but you can wade down the river to them.
Seeing a lot of geysers is another trick all to itself. To get the maximum geyser viewing time it helps to know the times and windows they are due and carry a radio set on channel 4.5. The gazers will broadcast times as the geysers erupt. Some may go on long enough for you to get there, or it may just be a case of knowing not to go an wait for that one and move on to the next on your list.