Couple of simple ones I do are press ups but with my feet on a pumped up yoga ball, takes a lot of effort to keep the ball still so feels like I use my core and shoulders more than normal press ups. Also been doing hand stand press ups against a wall. I put a stack of books under my head to control the way down and take a book away each time I want to make it harder. This can feel tweaky if I don't make sure I'm looking straight ahead and keeping my pelvis tucked in. If you have a bar you can hang on then there's lots of progressions for working towards front levers which will core you up; leg raises, levers with one knee to chest, split legs etc. Loads of yoga work outs on youtube too.
