3 year olds do like holidays, and going on planes...
1 adult to 2 children under 4 at center parks would be a nightmare - my parents treated us to it when my older 2 were 2 and 4; my sister who was there with her husband and 1 5 year old thought it was wonderful - I was on my own with my 2 (parents less than no help) and incredibly stressed trying to keep both of them alive at once, as they wanted to go in different directions on little water shoots and stuff - an utter nightmare.
I have 3 kids and take each to London for 3 nights the summer they are 7 (we live abroad, I justify it as cultural education, but it's a wonderful 1:1 trip, just planned around that child - and they all know they get to do it, but of course each time I take 1 and leave 2 kids.) We have also taken the older 2 to Berlin - youngest was delighted to get DH'S parents to himself for 3 nights and would have both hated the trip and ruined it for the older 2, who knew what they wanted from the trip, and it wasn't 3.5 yo attention span compatible.
I also took the your on a series of special day (morning) trips just before he started preschool - he wanted to go on a train, see deer, seedidinosaur bones, and go on escalators... The older ones weren't jealous :o
If you are used to juggling a 3 and a 1 year old alone, just the 3 year old will be a real holiday. The dynamic of 1 adult to 1 child is such a luxury, and your relationship will blossom. Your 1.5 year old will be delighted to have grandparents all to himself too - everyone wins, surely!