Sleep the first night on Horsetooth Reservoir, in the trough behind the hogback a few miles west of town, either at South Bay on the north end or out on the Inlet Bay cove. Larimer County runs both campgrounds and takes the reservation itself, so lock the site down before you book the van. Same day is possible, though a weekend between April and September comes with a two-night minimum. Then take the morning for Horsetooth Rock, the split sandstone tooth the ridge is named for, which stands directly above the water and is hiking only.
That leaves the afternoon for the reservoir, 6.5 miles of water with a swim beach at South Bay from Memorial Day weekend to about October 1 and a marina cove at Inlet Bay to put a paddleboard in.
Across the north arm, Arthur's Rock in Lory State Park is a second half-day climb worth the drive around, but Lory only has hike-in campsites, so the van still sleeps at Horsetooth.
Poudre Canyon
West of the reservoir, CO-14 turns up the Cache la Poudre, Colorado's only Wild and Scenic river, and the walls close in as ponderosa gives way to lodgepole. The canyon's overnight is Mountain Park, on the river about 40 miles northwest of town at 6,500 feet, with electric sites you can reserve and a first-come non-electric loop. Its spurs run 35 to 45 feet, long enough for a bigger rig if you would rather size up to an RV. Lower down, a footbridge crosses the Poudre to Greyrock, a bald granite dome you can climb in a morning and still be back at camp for dinner.
Pingree Park Road
For a night with almost nobody around, County Road 63E climbs south off CO-14 into Pingree Park, lodgepole along the South Fork with the Cameron Peak Fire's burn scar still showing on the way in. The overnight up here is Tom Bennett, ten first-come sites at 9,000 feet for $19 a night with no potable water, so fill the tank before you leave the highway. Its spurs run 15 to 25 feet and won't take anything longer, which is most of the case for a van on this road. Jacks Gulch, the developed campground on the same road, has stayed closed since the Cameron Peak Fire, so don't count on it.
Rocky Mountain National Park
Rocky Mountain National Park is southwest of Fort Collins, and the Poudre you drove up starts inside it. The park is Denver's to lead, so its timed-entry window and campgrounds live on the Denver campervan page.