I second Nicky's suggestion.
Find something that interests you, that would take several hours, and then start doing that thing. A good tip, is to select an activity in which your SO is unlikely to have any interest. Also, plan ahead. If challenged, be prepared to actually do (or appear to do) that thing. Here are some ideas (varying degrees of success, based on how inquisitive/curious/clingy your SO is):
- Pickup a part for the house/car/computer, in a remote location (an obscure part for the car, dishwasher, doorbell, etc. Particularly useful if you can pick up the part locally in advance, so you have a part to show). I do a lot of work on our cars. So, I could say, "I'm going to that junk yard across town, to look for any XYZ, because they have our year/model car".)
- Golf (as previously suggested)/Run/Bike Ride/etc. (as weather allows)
- Traveling a far distance to see something that you're interested in buying (but, you'd actually have to be looking for that thing - e.g. "There was a guy on the other side of town who had a copy of that vinyl album I wanted. Unfortunately, it was all scratched up. What a waste of time." Obviously, you can't use this indefinitely, and eventually you'd actually have to be looking for, and possibly, buy that item).
- Going to gamble (we are fortunate to have some open casinos nearby).
- Going to court ("I got a ticket for a broken taillight, license plate light, headlight, etc., and I have to show the repair receipt to get the ticket reversed." Ideally, it'd be something that your SO is unlikely to notice was 'broken' and that you then 'fixed'.
- Doctor/Dentist/Dermatoligist/Audiologist/etc.
- Traveling a far distance for your favorite Hot Dog/Taco/Pizza/etc. ("I have a taste for Marco's. I'm going to drive into the city to get some." Particularly useful if Marco's is near your destination and you can actually bring Marco's home.)
Good luck