We just got back from a week at RMNP. Had the wife and 8-mo old baby with me. Did mostly shorter hikes with them, then long ones by myself.
Lots of nice 1-3 hr hikes off Bear Lake Road. FYI, Bear Lake parking lot at end of the road fills up before 7:30am each day. If arriving after that, you have to use the park and ride halfway up the road (we didn't want to do this with the baby). We got there early most days, and got lucky arriving at 10am another day when we happened to catch someone leaving. This time of year, it is calm and blue skies earliest in the morning (sunrise at 6am), with clouds and rain rolling in after lunch. Even though the forecast calls for rain every day, it will probably be clear from 6-11am so get out early.
From Bear Lake parking lot, Dream Lake/Emerald Lake combo hike is a nice 2.5-3 hr round trip (including picture and food stops) with 600' elevation gain total over 1.8 mi each way. Very scenic with the mountains/glacier overlooking the lakes. Wife loved the scenery here and trail is in good shape.
Also from Bear Lake parking lot, can hike to Alberta Falls in 1 hr round trip - 1 mi each way with minimum elevation gain. Nice scenic waterfall with a few bridge crossings.
I did the Flattop Mountain/Hallett Peak hike by myself one day - took me just over 6 hours round trip with 3400' gain over 5 mi each way. I started early and ate an early lunch at the top of Hallett - incredible views of the park as you are basically sitting on top of the continental divide. Trail is well marked all the way to Flattop then you have to scramble and follow the cairns the extra 400' gain to Hallett. There were lots of elk (100+) hanging out near the top of Flattop just grazing so that was cool too.
We did the Fern Lake Trail with the wife/daughter one day - easy 1.5 mi hike to the Pool, which is a mini waterfall/gorge where the river cuts through the rock. The trail follows the river which is pretty nice - lots of trout fisherman on the river. Get there early if you want a spot at Fern Lake TH as it only has about 15 spots, but there are various turnoffs along the 1mi stretch of dirt road leading to the TH with 4-8 spots at each (this is where we had to park). We were gonna do the Cub Lake Loop which adds 600' gain and an hour or so to the return, but wife wasn't feeling up to it so we retraced the trail back to the Fern Lake TH.
Did some other short hikes around various lakes, near the alpine center at top of Old Fall River Road, etc, but Bear Lake Road area was definitely our favorite. Just be sure to get up and at em early to beat the crowds, get a parking spot, and have the best weather.
If you need any gear while in Estes Park, go to Estes Park Mountain Shop about 1 mile east of the main visitor center in town. They really took care of us. Rented an infant carrier backpack for $10/day, gave us trail tips, then returned there after I blew out my Asolo boots of 10 years to get the exact same model. They even gave me some local discount of 15% without me asking for anything. Really good folks.
Have fun and hope the weather works out for you. We had a blast.