There is no flower arranging in club activities

@EldircoraI have checked with a fellow helper and there if you make your own club you will be able to make a Flower Arranging Club. Please create the club using one of the existing club icons (the Gardening Club would be the logical choice here - or the 'romance' rose). Click on a plus sign in the 'Requirements' box. Under 'Choose Activity' select "Skill +2" and from that list select 'Flower Arranging'.  Note: It appears that your sim will need to have a skill level that is sufficient to create the club in this manner (apparently a skill level of 2 or more in that area). ...

BUT... unfortunately apparently though you can add the 'Flower Arranging of +2' as a requirement to be in the club... That does not actually give you an ability to add flower arranging as an activity that they get 'credit' for. You will need to go into the 'activities' box (click plus sign) and choose activities for your 'Flower Arranging Club' from that list, and you cannot add custom activities there. And, as you already noted, flower arranging is not on the activities lists. So yes, it is weird that though 'Flower Arranging' is added to the 'requirements' area as if it would be logical to create such a club, the choices to go with it in the activities are not what we would hope for. 

I agree with you that this would be a nice club to have in the list as a pre-set choice, or to at least have it in the activities to add to a custom club or to the Gardening Gnomes club! It would be a great suggestion to add to the suggestions/feedback thread, if you like: https://forums.thesims.com/en_US/categories/the-sims-4-feedback

Please let us know if we can be of further assistance. Thanks!