On the obstacle course side of the day:-
1. 2 small pallets, 3 planks, 6+ 6" nails: cover pallets with plywood; drill hole in each end of each plank big enough to take nail and matching hole at each end of each pallet.
2. Place plank (1) on pallet, drop nail thro' plank and pallet to hold in position and stick nail thro' other end of plank into ground.
3. Place plank (2)at other end of pallet and place on second pallet with nails thro plank/pallets
4. As 2 with plank (3).
Kids then ride up plank (1), across pallet on to plank 2, across plank (2) and 2nd pallet, then down plank (3). Make more difficult by angling planks and pallets
Get offcuts of yellow plastic mains gas pipe (they come in 9", 12", 18" and 24". Get more planks and bolt to pipes: this creates seesaws (teeter-totters to you) of increasing difficulty.
Get couple of milk crates with empty 1 and 2 pt plastic milk bottles, fill with sand (not water) and play pick up bottle from first crate (on its side) and put on second. Make more difficult by graduating to upside down washing up bowl (or pallet) and finally picking up off floor and replacing. If you have an irritatingly good kid, make them pick up bunch of car keys.
Get length of 15mm overflow pipe; cut top off 2L pop bottle at point where it starts to curve; put bottle hole onto 6" length of pipe into and the bottle bit then acts as bowl into which you can put one of those plastic practice golf balls. Kids then do a relay, riding one-handed up to cone and riding back without ball falling out before handing on to next rider.
My planks, pallets and plywood were picked up from civil engineering site - they were all slightly damamged and were being dumped, as were some boxes used for concrete shuttering and which I use with planks when pallets become too tame for the better kids.
You can also place the pallet(s) under the seesaw base to make it more challenging
Mix in with bike rodeo course layout (cones, etc).
Use high jump set for limbo competition - except you gradually lower the bar.
Have a nice day, now.