If I'd stayed home with a 3 & 5-year old (as perhaps you are doing), I'd have gone nuts -- paying for childcare was well worth it to (and possible for) me. And even with that, I did dread being indoors at home with my LO (and I have just one, so the inter-sibling rivalry was zero. Of course, so was the inter-sibling play). A big fan of going outdoors, but the weather can make it hard (and it's well and good to say, "No bad weather, only inadequate clothes," but I find (a) lightning storms, which are not infrequent around us in summer time, (b) 90+ degrees in ditto humidity, and/or (c) persistent biting bugs to be pretty challenging and arguably not worth navigating with little kids, ditto 40-and-raining. All temps are Farenheit :)! ).
Besides the things folks (including you) have already rattled off -- swimming pool, kids' museums/science centers, zoos, aquariums, libraries, we would also go to the local mall, which is pretty woefully empty, ideally at times when it was quiet, just so DS could wander around. And to big home improvement stores, which are common in the US (not sure about where you are), again ideally at the time when the store was quiet and DS could e.g. pretend to drive the lawn tractors without bothering anyone. Some of our local town organizations also occasionally offer stuff like crafts or drums. I'm not much of a crafter myself and preferred to outsource that when feasible, but little bits here and there are tolerable IMO. And baking, as others have suggested (we went through a phase of making banana bread every. single. day. and gave away a lot of loaves. But if I just left DS (~3) to stir, etc., each ingredient after we added it, just putting the dough together could easily consume an hour. Of course, having 2 probably makes that trickier (but perhaps each could mix their own batch of dough in a separate bowl?).
GL. In my experience, it does get better as they get older.