If it's a mini-adventure with one or more night (s) camping / bivouac, pack clothes for the cold, wind and rain so that you're prepared for all eventualities. The stove and its fuel, meals, cutlery, a fire stone, a knife... The tent, the sleeping bag, the groundsheet, a headlamp, a small bag for dirty laundry guarantee a cocooning spirit. A first aid kit * in case of ailments and an additional phone battery, useful if you cannot disconnect completely.
For a bike trip, remember to clean and oil the chain as well as the bearing points such as the crankset, the derailleur and the disc brakes. Also check the screws, the tension of the brake cables or the wear of the disc brake pads.
Finally, you want to go kayaking, stand up paddle boarding, packrafting, surfing or climbing, a hike with pulka or a fishing session (you want to do a lot of things) but you do not have the material, maybe a friend can lend it to you. Not having the equipment doesn't mean the mini-adventure is impossible. Check out your network! Better to borrow or possibly rent equipment rather than impulse buying.