All based off vanilla game rather than Open RCT but likely to be similar/the same:
One thing to bear in mind is scenery limits- after a certain number of items are selected by any means (individually with advanced selection or because they are in selected groups) you can't pick any more- also there's an original game bug/problem common to several of these types of object-based selections in games of the time- sometimes even items you could select don't appear to use in-game (even once you've researched all scenery groups- which is another tip- if you want to use them from the start make sure the scenery group is already invented!), I think this is because they somehow drop out of the selection windows if there's too many 'things'- I'm sure the editor allows you to pick more objects than actually make it to the game.
It's very worth noting that some scenery groups are huge and you won't have need to use everything in them a lot of the time, so another scenery picking tip:
It's a bit trial and error- sometimes you won't have an item if it's scenery group is missing, or you'll deselect items if you deselect a scenery group. I think you can go through the item-by-item list (yes this does take ages!) with advanced set to on and tweak the items that the selected groups do/don't give you- in other words pick a scenery group to be active but then remove some of its items in the full list. Also there's a misc section for picked items not in the selected scenery groups, this may be where some scenery picks go missing so to minimize 'missing' picks if you're using a lot of a particular scenery group it's worth having that group selected (thus using the max number of scenery group picks for the ones most used) so it doesn't have to put those items in the misc tab as it will put them in that scenery tab.
I keep thinking one day I'll make a list of what's in each group vs the full list but I've not got round to it yet! Which is counterproductive laziness/procrastination as it would save so much hassle and repeated work. Likewise was going to test the missing items problem to see if there was a rule to avoid it, also not done yet....
It's time consuming (you'll need a long checklist for starters) but it's worth testing/checking all your selected scenery items from the scenario design are in the game before playing it. If not you may need to tweak your selections, removing a few or refining your groups to optimize your desired choices. Backup first!
Another tip- you can place scenery in the designer even if it's not available to players until later in the game due to scenery group research settings- this is useful, it also means you can set up the possibility of players 'improving' an area/ ride's theming by using one or two items in the design stages (in-game if you want a scenario with rides) then leaving the player to research theming to be able to add the others. What I've not tested is if you can place scenery in the designer then remove it from the available scenery altogether, you'd have to experiment, it could be useful if you can as you'd free up some in-game slots unless it still uses them re the missing scenery problems.
One thing I'd really like to see in the eventual ground-up rebuild of the software is this missing scenery items selections bug being fixed and hopefully removing the issue altogether by having all scenery always available (it's a real pain spending hours- and it can be many hours- choosing the scenery you need, checking it and of course that head hitting moment of ah, I haven't got X bit of scenery I should have included/need and now I can't change what's available. It's also a bit of a brain fryer trying to work out exactly what you will need for a particular park project before you build it as it can be such a huge number of parts.)
This is an understandable limitation of the original game but hopefully a full code rebuild could fix it, hopefully someone will figure out how! Plus having some global scenery that isn't so, shall we say, predictable (ooh a boomerang for Australia....), something a bit more creative!
Finally, maybe the OpenRCT devs could come up with something that improves scenery management even before any ground-up rebuild, although it would have to not break any existing design setups.
If anyone else has any scenery selecting experience/tips/issues etc to share please do!