Have a partner now, but spent a long time single and am also a teacher so had holidays when others were walking. I'm one that likes my own company though, and have happily travelled alone etc. I also have a dog. And a big extended family with many nephews/nieces forever hankering after a sleepover if things get really desperate!
I think it very much depends on personality type, obviously whether you like your own company or not, but also how much structure/plans you want in your free time. My best friend is like you, doesn't particularly enjoy doing things alone but does always want to be doing things and to have a plan. I'm much more of a potterer, and can easily and enjoyably spend a day doing not very much - bit of a lie in, few extra cups of tea whilst reading, walk the dog, walk to the village to get lunch and buy ingredients for dinner, come home, read some more or fanny about online, few jobs round the house, walk dog again, make dinner, eat in front of Netflix and bed. Pretty perfect day in my book, though I'm sure hellish for some people.
Things that I've enjoyed doing alone - yoga classes, cinema, sitting in a coffee shop with a book, shopping/browsing - especially book shops but not averse to looking at shoes and make up either. Book to get a massage, or manicure etc - usually easier to get in mid week and combined with a stop for lunch then it kills a good few hours. I've also joined meetups in the past when I've wanted to do something that was different from what my friends are into (we either visit each other for tea/coffe, go to the pub or go for a meal) - stuff like going to lectures or going on guided walks or the theatre. I like cooking too and can happily spend the day pottering in the kitchen. Swim and sauna.