For me, nothing beats batting on grass (if you play on grass this is btw). Whether this be in a match scenario, an actual centre wicket practice or using a mobile batting cage on the square. Not only is it more realistic compared to the usual flat indoor surface, it stops me getting them annoying green "cherrys" on my bats which I can never seem to remove.
We are fortunate at my club to have a cage and a fielding net which allows us to set up two lanes outdoors or block off one half of the outfield (either legside or offside). This free's up some of the fielders, as even though fielding is important, for me turning up after work to stand in a field in an uncompetitive environment for an hour and a half isn't the most fun.
This set up, also means that two groups can be set up with one taking on an actual fielding drill and the group netting can carry on. Bowlers/batsmen then rotate out of the fielding group and the netting group throughout the session as fatigue sets in or some variation is required.
It might not quite beat a fielding net or a mobile cage but if either of these are not available options then try and make a line of kit bags for those who have turned up to block off some of the field. Even if it saves you one or two fielders they could head off to take some catches, do some ground fielding work etc etc. It might not stop the ball at all times and of course people can hit over the top...
Some things to consider... Dont put the fielding group somewhere they are likely to have balls whacked at them from the netting group and vice versa really as some people hitting high catches do like to put snow on them and sometimes hit them a bit too far. Raining bombs on a set of bowlers or batsmen is not the way to go.
Set a limit on the number of bowlers if you decide not to do overs. We've found that 2/3 seamers and one spinner is typically the best option if you can. Allows the batsmen to face up whenever ready and if the flow is good enough the seamers are not too far off their usual "over" time when they've bowled 6 balls. Stops bowlers getting too fatigued and batsmen should have no complaints that they've only faced a few balls.
I'd agree on the point of the session not just being a 1xi - good for development of other players and the club atmosphere if you can have as many down as possible.