Thanks for your question regarding your 2 year old girl Sindee who has been both an indoor and outdoor cat! It certainly sounds like you have enriched her current indoor enviroment very well, so don't feel bad for her - it is just the way it is and you are doing your best for her.
Firstly - in answer to your question regarding the cottage next week. I would definitely advise that she stays indoors during your stay here. Because this will be a completely new environment for Sindee with all sorts of new and interesting objects and smells - and potentially some 'threatening' smells of other pets that have been in the cottage, it is best that she stays inside. As this will be a new and interesting environment for your girl - being inside the cottage will be more than enough excitement for your girl for the week. Furthermore - I would be concerned that introducing her to the outdoors in this new environment could through her off a little bit and there is the worry she could wander off and not find her way back to you again. So yes - I would recommend you don't let her outside.
You definitely have a number of enriching toys in your home which is great. Continue to buy cheap but exciting toys for her. Glittery, scrunchy, cheap catnip toys are perfect. Having a range of different toys and interacting with her with them is great. You could also buy a cat food activity ball as this is a great way to keep her entertained as well. It is also a good idea to have a very tall cat tree that she can climb on, scratch on, play on and sleep on. Have a number of cozy places for her to relax up her. This could mean making a little bed for her on top of a wardrobe, or you could install some screw on wall shelving as well. There are some great ideas for these online here: http://www.moderncat.net/category/products/climbing-perching/
So long as you are interacting with her as much as possible and that she has plenty to keep her entertained while you are out - she will be more than happy to be an indoor kitty I'm sure! So definitely don't feel guilty.
I hope this helps settle this issue for you. Best of luck with your girl and give her a pat from me. Don't forget to click a smiley :-D face if this has been helpful.
Bonuses & positive feedback (5 STARS OR AN EXCITED FACE :-D ) are always welcomed and very much appreciated