I have this exact age gap. They're 8 and 4 (nearly 5) now. The park was and still is the easiest win for them both. Preferably a park with space for DC1 to play football or basketball, and play equipment for DC2.
When DC2 was a baby I obviously just took her to whatever DC1 was doing. It's when the youngest starts walking that it becomes a bit trickier I think.
For a while, soft play was doable although better if DC1 had a friend - then he'd been happy going around with his friend while I stayed with DC2. As she got older she wanted to go with him on the big climbing frame and there was a tricky stage when she needed me to go with her to help, i was relieved when she got big enough to go without me!
As they get a bit older, indoor play with inflatables or trampolines can work well, as they both enjoy it and it's slightly easier than soft play to look after the younger one while also keeping an eye on the oldest.
Swimming in a shallow pool where the oldest can touch the bottom.
Days out are easy enough, DC1 is getting a bit old for the farm park type places aimed at 0-6 year olds, but there's plenty they both enjoy, zoos, theme parks etc.
We took them to LEGOLAND when DC1 was 4 and DC2 was about 7 months old, obviously DC2 couldn't do anything so it was boring for the parent stuck looking after her, but the day was for DC1 and he really enjoyed it so I'm still glad we went. We went back recently - 8 and 4 were the perfect ages as they both loved it, DC2 couldn't do ALL the rides but was able to do most of them.
Like a PP I do relish 1-2-1 time with each of them, it's just easier and nicer to enjoy their company individually... I recently took DC1 to see a PG film which we both enjoyed but wouldn't have been suitable for DC2.
I think it's a nice age gap tbh although mine are 4 school years apart (because of where their birthdays fall) and I would have preferred 3, but never mind!