Which hikes in rocky mountain national park are easy up to about an hour round trip and are scenic (with waterfalls, lakes)? Coming from a lower elevation, is it ok to do hour long hikes on the day we get there? Or is it too much?

"It depends"

For me, an hour long hike on the day you arrive at elevation is perfectly fine and a good way to help aclimate. But we don't know (nor need to know) your fitness level, preexisting conditions, regular exercise routine, etc, etc. So for me, more important than the time may be the effort. If that hour long hike is 15min per mile pace up a steep incline, and your heart rate stays above 150bpm, then that's a different type of hike than a leisurly pace and your HR never gets above 100. Those #s are just for the sake of discussion..... If your daily routine at home is high intensity, then that's a different answer than if you rarely exercise vigorously....

For hiking, I like to use alltrails.com, both on the web (for planning) and app (for downloading maps & real time route tracking).

I always recommend talking to a park ranger at the visitors center and they can tell you which trails would be best for you and give you a map.

Yes, "it depends". But check out this site:
https://www.rockymountainhikingtrail...ional-park.htm

I've hiked pretty much every trail he lists, multiple times in most cases, and generally tend to agree with his assessment. On his grading system anything under four I'd consider easy-ish. Four to seven is moderate, above is difficult. Those are just guidelines.

And here is the official RMNP page:
https://www.nps.gov/romo/planyourvisit/hikes.htm

It's OK to do a longer hike upon arrival if you take it easy, stay hydrated, and don't burn yourself out. In fact it's usually good to go up to a higher elevation than Estes (assuming that's where you are staying), then coming back down for the evening. "Climb high, sleep low" is the mantra.