Games like IncrediMarble
Games like IncrediMarble include Garry's Mod, People Playground, House Flipper, Planet Coaster, and Townscaper. We matched them by the tags players give IncrediMarble on Steam (Sandbox, Relaxing, Idler, Building, and Casual): closest matches first, and among equally close ones the better-known games.

Garry's Mod
Shared: Sandbox · Building · Physics

People Playground
Shared: Sandbox · Physics · Simulation

House Flipper
Shared: Simulation · Relaxing · Sandbox

Planet Coaster
Shared: Sandbox · Building · Simulation

Townscaper
Shared: Relaxing · Sandbox · Building

Stardew Valley
Shared: Relaxing · Sandbox · Simulation

Tiny Glade
Shared: Relaxing · Sandbox · Building

Cities: Skylines II
Shared: Sandbox · Building · Simulation

Space Engineers
Shared: Sandbox · Building · Simulation

Potion Craft: Alchemist Simulator
Shared: Relaxing · Simulation · Sandbox

Hozy
Shared: Relaxing · Sandbox · Casual

Paralives
Shared: Building · Simulation · Sandbox

My Dream Setup
Shared: Relaxing · Building · Sandbox

The Sims™ 4
Shared: Building · Sandbox · Simulation

Kerbal Space Program
Shared: Sandbox · Simulation · Building

Totally Accurate Battle Simulator
Shared: Sandbox · Simulation · Physics

SUMMERHOUSE
Shared: Sandbox · Relaxing · Casual

Cities: Skylines
Shared: Building · Sandbox · Simulation

Poly Bridge
Shared: Building · Sandbox · Simulation

Prison Architect
Shared: Sandbox · Building · Simulation

PC Building Simulator
Shared: Sandbox · Simulation · Building

Megaquarium
Shared: Sandbox · Building · Casual

Slime Rancher 2
Shared: Relaxing · Simulation · Singleplayer

Farm Together
Shared: Relaxing · Simulation · Casual